The seasonality and factors associated with Cryptosporidium infection were assessed in a cohort of HIV-infected patients in Los Angeles County to better define the epidemiology of cryptosporidiosis among individuals with HIV. Data were analysed from a cohort of 4247 patients [ges ]13 years of age with HIV infection enrolled from four outpatient facilities in Los Angeles, 1990–6. Cryptosporidiosis was diagnosed in 120 (2·8%) patients. Among the 1296 individuals with complete follow-up until death, cryptosporidiosis occurred in 69 (5·3%). The seasonal rate of cryptosporidiosis showed a modest bimodal trend with the highest rates occurring in March–May and September–October. There was no difference in the rate of cryptosporidiosis for the periods of heaviest rainfall (December–March) and low rainfall (April–November). Infection rates were higher among males (1·59 per 100 person-years) than females (0·92) and lower in blacks (0·98) than other racial/ethnic groups (1·80). A significant trend of decreasing cryptosporidiosis was observed with increasing age, with the highest rate (2·34) in the 13–34 year age group. A strong association between cryptosporidiosis and CD4+ count was noted. These data suggest that cryptosporidiosis among HIV-infected individuals in Los Angeles County exhibits a modest spring and fall seasonality. This pattern of occurrence of cryptosporidiosis appears temporally unrelated to local rainfall patterns. Our findings suggest that HIV-infected men, individuals in younger age groups and those with CD4+ lymphocyte counts <100×106/l are at increased risk of cryptosporidiosis. Blacks with HIV infection appear less likely than other racial/ethnic groups to be diagnosed with Cryptosporidium infection. These results may provide insight into possible routes of transmission and sources of cryptosporidiosis infection in individuals with HIV.