Cryptosporidium parvum may cause severe, debilitating diarrhea in patients with AIDS. Recent anecdotal reports have suggested that hyperimmune bovine colostrum may be effective. We conducted a double-blind, controlled pilot study of hyperimmune bovine colostrum for diarrhea due to cryptosporidiosis in five AIDS patients. The patients were randomized to receive either hyperimmune or control colostrum by continuous nasogastric infusion for 10 days. All stools were collected, graded, and weighed, and the concentration of oocysts excreted was determined daily. One of the three patients treated with hyperimmune colostrum had a reduction in diarrhea and in the concentration of oocysts excreted. A second treated patient had a modest decrease in the concentration of oocysts excreted. Two patients who received control colostrum also had decreases in the volume of diarrhea but no change in the concentration of oocysts excreted. We conclude that hyperimmune colostrum with high titers of specific anti-Cryptosporidium antibody could be effective in treating patients with cryptosporidiosis. However, more studies of cow colostral immunoglobulin need to be performed so that the efficacy of this treatment can be assessed more thoroughly.