I’ll start. This weekend’s 70.3 Belgrade had 493 finishers out of 789 listed athletes (37.5% DNF).
9 degrees Celsius, pouring rain all night and morning and wind answers why. Toughest conditions out of the 196 triathlons I have taken part. I’d never seen so many people walk out of transition with their bikes and bags before a race. I finished but still suffering from hypothermia.
Sounds like a lot of DNS’s PLUS the DNF’s - defrost/thaw out and recover well
I DNF’d Aix 70.3 in 2018 (south of France in May!) after a freak-ish, very cold rainstorm which hit me on a fast part of the bike course when I was dressed for the weather we’d had at the race start. I managed to start the run, still shivering uncontrollably and was eventually taken from the course to a large room at T2, where I discovered lots of other people shivering wrapped in silver blankets. There was some solace in company at least! Checking the results, only 1477 out of 2298 people finished (can’t tell which were DNF vs DNS).
According to his Rules, if you don’t win the Backyard Ultra, Laz gives you a DNF, so the DNF rate approaches 100%
Two races always mentioned in this is some super hot N/A race in 2015/2016 maybe CDA not sure, and Frankfurt 2019. Frankfurt 2019 had 3000 starters and 2000 finishers so 33% DNF and a day reaching 37C during mid bike to mid run for AGs.
I have never seen so many people in medic tens/just sitting on the side of the road even on the bike portion where you atleast have the wind.
IM Chattanooga 2016
Daytona 500 20/40 completed it.
Lake Tahoe 2013 was up there. 24F / -4.5C air temp at the start. 21% DNS and another 20% DNF of those remaining.
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IM Wisconsin 2022 I dnf’d due to hypothermia. It was in the 40s and raining all day and I was not prepared for that unfortunately. I did learn a lot from that race though. I don’t remember the percentage that dnf’d but it was much more than a usual IM
Thanks 
Yes I counted all those DNS altogether as DNF since there is no further info available. Although I would consider those who DNS but had already checked in also as DNF (just wiser people).
At first I was a bit surprised when the guy in the rack behind me took his bike and walked while I was clipping my shoes to the bike. Then I saw another one, and another one. There was even a queue to get out.
I also raced IM Frankfurt with 40°C and it was brutal but you could always stop at an aid station and sink your head in the ice. When you’re shivering there’s no turning back.
I took a taxi to the hotel straight after I crossed the finish line in a thermal blanket, went into the sauna even before getting to my room and spent 30 freaking minutes at 80°C before I began to feel hot and started sweating a bit.
IM Waco 2022: 556 starters according to slot allocation table (already very small), 380 finishers = 32% DNF. It was 95F, with bad roads and crosswinds on the bike. Lots of folks totally wrecked after T2 and bailed.
On the plus side there were 54 WC slots for those 380 finishers…
I think you will find that a large % of that 37.5 DNF you site is actually DNS. Be pretty hard to tell how many, as a lot would have pulled the plug that morning after check in I bet…
The largest % of DNF I can recall is USTS 1983 Seattle. Water was 51 degrees in the Puget Sound, and there were no wetsuits among the 400 or so starters. Believe less than 50% actually finished, most in the shortened by half swim of course, the rest probably on the bike getting chilled down from the cool air temps…No one died…
Next would probably be the late 80’s big money and TV race the Oceanside Triterium. It was invitational only, so just pros and elite age groupers. Surf was 10ft hitting top of pier, and it was a two lap swim. Since we did not cancer swims back in those days(for anything really) the race went on as scheduled with both laps of the swim. Lots of pros did not finish that day out of the water, probably close to a 40+% DNF rate… No one died…
Embrunman. I think typically something like 25-30 percent dnf, otn (over time limit) or dns.
I think a good question is name a race that every year has a high rate of dnfs.
1984 Lake Placid I DNFed with hypothermia
No wetsuits back then
Speedos and a tank top on the bike ala Dave Scott in 45 degree weather 
My wife did IM Frankfurt one year when it was 38 on the run and there were just bodies everywhere. That was about 10/11 years ago.
The year it was no wetsuit swim was pretty horrific as well.
She also did IM Switzerland the year it rained/hailed the whole day, that was pretty horrendous for DNFs as well. A lot didn’t start the run as it was too cold.
Texas 70.3 one year (2018?) there was a storm mid-race and they cancelled the event. Only finishers were in the sub 5hr crowd.
Bala one year had the most horrendous swim and were bring swimmers back to shore straight after the start. That must have been one of the worst % finishers.
official finishers yes. if you were on the run course you still were considered a finisher though and got a medal, AWA points, etc.