Objective: To study epidemiology and possible risk factors associated with the development of cryptosporidiosis among European patients with AIDS. Methods: An inception cohort of 6548 patients with AIDS, consecutively diagnosed from 1979 to 1989, from 52 centres in 17 European countries was studied. Data on all AIDS defining events were collected retrospectively from patients' clinical records. Kaplan-Meier estimates, log rank tests and Cox proportional hazard models were used to examine for possible risk factors associated with cryptosporidiosis. Results: Cryptosporidiosis was diagnosed in 432 (6.6%) patients, 216 at time of the AIDS diagnosis and 216 during follow-up. The probability of being diagnosed with cryptosporidiosis at AIDS diagnosis was significantly lower for intravenous drug users (1.3%) than for homosexual men (4.1%) and for patients belonging to other transmission categories (4.0%) (p < 0.001). The probability was also higher for patients from Central Europe compared with patients from South Europe (4.1% versus 2.5%, p = 0.005). The rate of developing cryptosporidiosis after the diagnosis of AIDS was 3 per 100 patient years of follow-up. The rate was significantly lower for intravenous' drug users than for homosexual men (relative risk 0.34, 95% confidence limits 0.22-0.54) and for women compared with men (RR 0.43 (0.21-0.87)). The risk was higher in North Europe than in South and Central Europe. In a multivariate analysis only transmission category remained a significant predictor for the development of cryptosporidiosis. Conclusion: The development of cryptosporidiosis in AIDS patients may be associated with sexual risk behaviour.