Cryptosporidium parvum is a protozoan parasite that causes severe enteritis in patients with AIDS for which there is no effective therapy. Paromomycin is a nonabsorbable aminoglycoside that is effective in the treatment of other intestinal protozoa. The ability of paromomycin to inhibit C. parvum infection of a differentiated human enterocyte cell line was evaluated in vitro. Paromomycin concentrations ranging from 50 to 5000 micrograms/ml inhibited infection at 24 h in a dose-dependent fashion. Concentrations greater than 1000 micrograms/ml, which are theoretically achievable in the bowel lumen, inhibited infection by greater than 85% (P less than .001). Prospective clinical trials of paromomycin for the treatment of cryptosporidiosis in patients with AIDS are warranted.