Cryptosporidiosis is an important cause of diarrhea. We identified 95 patients with cryptosporidiosis over a 6-year period in our county hospital system, including 9 children and 86 adults infected with the human immunodeficiency virus (HIV). Risk factors included male-to-male sexual practices and Hispanic race. Diarrhea, weight loss, and gastrointestinal complaints were the most common symptoms at presentation. Among the HIV- infected adults, 20 (23%) developed biliary tract disease. Biliary involvement was associated with low CD4 counts. Treatment with paromomycin and antimotility agents was effective in reducing diarrheal symptoms in 54 of 70 (77%) patients with the acquired immunodeficiency syndrome (AIDS), although there was a high rate of relapse. Paromomycin did not prevent the development of biliary disease. Biliary disease responded to cholecystectomy or sphincterotomy with stent placement. Though often a cause of morbidity, cryptosporidiosis was only rarely the cause of death, even among patients with HIV. Cryptosporidiosis continues to be an important medical problem even in developed countries. Current methods of prevention and treatment are suboptimal.