The influence was examined of ultra-long-distance running (1000 km race lasting 20 days) on changes in serum lipids. The 110 participants received two types of diet, a conventional Western diet and a wholesome vegetarian diet. Of the 55 finishers the serum concentration of total cholesterol, low density lipoprotein (LDL)-cholesterol, apolipoprotein B and triglycerides decreased significantly during the first 8 days of the run, but rose again towards the end of the race without reaching pre-race levels. The high density lipoprotein (HDL)-cholesterol increased initially but decreased in the final days of the run. The values for apolipoprotein A-I were not correlated with HDL-cholesterol. The free fatty acids and free glycerol showed marked increases (five times the prerace concentration), falling towards the end of the run. Changes in serum lipids showed no correlation with changes in body mass. Similar changes were observed in both dietary groups.