We prospectively evaluated the safety and efficacy of colostrum-derived bovine immunoglobulin concentrate in the treatment of diarrhea caused by Cryptosporidium parvum in patients with AIDS. A total of 24 patients with severe chronic diarrhea and AIDS were stratified to one of three cohorts: (1) C. parvum infection alone (n = 16), (2) C. parvum and a second opportunistic infection (n = 4), and (3) idiopathic AIDS enteropathy with no identified source of infection (n = 3) or an untreatable opportunistic infection other than C. parvum (n = 1). All patients were treated with bovine immunoglobulin concentrate for 21 consecutive days. Patients in cohort 1 were randomized to receive the medication in powder or capsule forms, whereas all patients in cohorts 2 and 3 received the powder form. The primary end point was change in mean daily stool weight. Secondary end points included change in stool frequency and body weight, as well as clearance of C. parvum oocytes as analyzed on stool microscopy. Patients with C. parvum who were treated with bovine immunoglobulin concentrate in powder form experienced a significant decrease in mean stool weight, from 1,158 +/- 114 g/day at baseline, to 595 +/- 63 g/day (p = 0.04) at the end of treatment, and 749 +/- 123 g/day (p = 0.03) 1 month after completing treatment. Stool frequency decreased from 6.6 +/- 0.6 bowel movements per day at study entry, to 5.4 +/- 0.7 during treatment (p = 0.04), and 5.4 +/- 0.9 during observation (p = 0.12). Patients who received bovine immunoglobulin concentrate in capsule form and patients without C. parvum (cohort 3) showed no improvement. No serious side effects were observed, and the medication was well tolerated. Thus, bovine colostrum immunoglobulin concentrate, in powder form, appears promising in the treatment of severe diarrhea caused by C. parvum. The optimal dosage, duration of therapy, and overall efficacy need to be determined in placebo-controlled trials.