Most of the 5423 entrants in the Melbourne 1980 Big M Marathon were non-elite athletes. A study of a stratified random sample of 459 entrants (which represented a 42% response rate) found that, while entrants reflected the community standards of disease, they pursued healthier lifestyles. Preparation for the marathon led to a number of positive changes in the health standard of runners. The principal negative consequence of marathon training was the high rate of musculoskeletal problems (30%). Before the race, only 4% of participants had an adequate fluid intake; 33% had pre-existing problems, mainly involving muscles and joints (63%) and viral or gastrointestinal illnesses (41%). These entrants had a 60% less chance of finishing the race. Symptoms during the race were reported by 92% of entrants, but most of these were not serious; only 6% of entrants were unable to finish the race. The pattern of symptoms after the race was similar to that during the race; 50% of these resolved within three hours. Ninety-seven entrants (2%) required medical attention during the race. Serious problems were rare (only in three entrants), and no runner required admission to hospital for longer than 24 hours. Entrants were at greater risk of requiring medical attention or experiencing problems during and after the race if they had a shorter preparation (less than two months), ran fewer kilometres per week (less than 60 km/week) in the last two or three months before the race, and had performed fewer long training runs (more than 24 km).