All-terrain vehicle (ATV) accidents cause 7000 injuries and 20 deaths per month. In this prospective multicenter study comparing three- and four-wheelers, data were obtained on all ATV accident victims treated at 33 participating institutions in 13 Western Michigan counties from July-November, 1986 (N = 143) and on 51 accidents from the preceding 18 months. Of these 194 accidents, 29 per cent involved four-wheelers. In this 78 per cent male population (mean age 21), 23 per cent required hospitalization. One half of these victims sustained a fracture or dislocation and 14 per cent needed surgery. Thirteen per cent had serious neurologic injury. Average hospital bills were $294 for outpatients and $7669 for inpatients. Two thirds of the patients had some disability. Three fatalities were recorded. Analysis of the data showed the following: (1) the three-wheeled ATV design is significantly more unstable than that of the four-wheeler (P less than .001); (2) though more stable, the four-wheeled ATV was still associated with frequent accidents as severe as those involving three-wheelers; (3) riders under age 16 were more likely to be using their ATV improperly (P greater than .05) and had more severe injuries with a higher hospitalization rate, and (4) these ATV riders had inadequate training and protective gear. Lastly, recommendations are made that ATVs need their design flaws corrected for safer use, and that mandatory minimum age requirements, rider education, and helmet use should reduce injuries.