Cryptosporidium parvum, an intracellular protozoan parasite, is a significant cause of gastrointestinal disease worldwide. Transmission can occur from an infected person, animal or fecally contaminated environment. The clinical manifestations of cryptosporidiosis are dependent on the immunologic state of the host. Infection among immunocompetent hosts results in diarrhea that is typically self-limited. In immunocompromised hosts, however, the infect I on may be protracted and life-threatening with no reliable antimicrobial therapy. In transplant patients, a Course of antimicrobial therapy along with concurrent reduction in immunosuppression optimize immunologic status and may potentially lead to resolution of the infection.