A prospective cohort study was designed to describe the patterns and to determine the factors associated with the risk of rectal prolapse in a commercial swine herd in California, USA. Thirty (1.0 per cent) of 2862 pigs prolapsed between 12 and 28 weeks of age with the peak incidence occurring in 14- to 16-week-old pigs. The overall prolapse rate was 9.1 cases per 100,000 days at risk. Prolapse rates were highest during the winter and autumn months. Other factors associated with an increased risk of prolapse were maleness (relative risk 2.3) birthweight less than 1000 g (relative risk 3.4) Yorkshire boar A (relative risk 2.8) and dams of litter number 1 (relative risk 14.9), 2 (relative risk 8.2) and 3 (relative risk 9.8). No evidence was found to support the hypothesis that diarrhoea and coughing are factors associated with a risk of prolapse.