Objectives: To determine the proportion of individuals who repeated nonfatal suicidal behavior within 12 months of an index episode, to investigate the timing of repetition, and to investigate risk factors associated with repetition and their population impact. Method: We carried out a prospective cohort study (1997-2002) in 4 large hospitals in North West England. We included subjects aged 15 years and over who attended with "self-harm" (an act of intentional self-poisoning or injury irrespective of the apparent purpose of the act). Following the episode, a standard assessment form, which included detailed demographic and clinical data, was completed by a clinician. Results: 9213 individuals presented during the study period. The incidence of repetition within 12 months of the index episode was 13.6% (95% CI: 12.9% to 14.4%). The median time to first repetition was 73.5 days (interquartile range, 20 to 187 days). One in 10 subjects repeated within 5 days of the index episode. Independent risk factors for repetition included previous suicidal behavior, psychiatric treatment, being unemployed or registered sick, self-injury, alcohol misuse, and reporting suicidal plans or hallucinations at the time of the index episode. The combined population attributable fraction (an indicator of the potential population impact) for these variables was 65%. Conclusion: The repetition of suicidal behavior is common and occurs quickly. On a population level, our study suggests that the most important strategies to reduce repetition might include primary prevention of suicidal behavior, targeting psychiatric illness, and tackling social factors such as unemployment. Specific interventions may be required for individual subgroups.