A population-based cohort study was done to estimate the incidence of different types of injuries and to evaluate certain risk factors in an urban slum; 4333 slum dwellers in Madras city in India were randomly selected by cluster sampling, of whom 1.7 per cent were lost during the 12 months of follow up. The cumulative injury incidence for 12 months for all injuries was 127 per 1000 persons (95 per cent confidence interval 117-137); for males 137 per 1000 and for females 118 per 1000. Incidence of unintentional injury was 121 per 1000 persons. The incidence for road traffic injury was 16 per 1000 persons, for household injuries 57 per 1000 persons and for injury at place of work 19 per 1000 persons. The relative risk of males to females for traffic injuries was 3.04 and for household injuries was 0.39. The relative risk of traffic injuries among adult males who reported daily alcohol consumption was 2.26. The incidence of injury is high in an urban slum and it is a priority health problem. This study has identified groups of people who are at high risk for injuries and who may need specific protective measures.