To evaluate the characteristics of runners with exercise-associated collapse (EAC), we studied the time of onset of collapse, rectal temperatures, cardiovascular status, and incidence of readily identifiable medical conditions in 46 male athletes who collapsed during or after a 56-km ultramarathon footrace run on a cool day. Data were compared with 65 control runners who did not collapse in the same race. Weight changes during recovery were studied in a subsample of both groups. The majority (85%) of runners with EAC collapsed after they had completed the race; rectal temperatures (38.5 ± 1.3°C, mean ± SD; range 35.5–42. 0°C) and supine heart rates (87.5 ± 17.2 rain-1; range 60–138) were only modestly elevated. Postrace serum sodium concentrations, changes in plasma volume, and mass during recovery were not significantly different from values in control runners. We conclude that: (i) most cases of EAC (85%) occur after the finish line; (ii) runners collapsing during the race are more likely to have a readily identifiable medical condition than runners collapsing after the finish line; (iii) runners collapse most frequently near cutoff times for medals and race closure times; and (iv) 16% of EAC casualties and 19% of control runners have identifiable biochemical abnormalities.