Interviews were conducted with 46 people injured on a newly opened water slide in Washington State during the summer of 1983, and 46 age-matched controls. The injuries included nine concussions and eight spinal fractures. The odds ratio associated with being 10 per cent over ideal body weight, adjusted for age and sex, was 1.6 (95% C.I. 1.1-2.5). Three of eight people with spinal fractures were riding with another person between their legs, compared with two of 38 other injured riders (OR = 10.8, 95% C.I. 1.8-63.5).