Small intestinal biopsy established the diagnosis of cryptosporidiosis in a 6-month-old male infant with severe combined immune deficiency. Scanning and transmission electron microscopy revealed the ultrastructural details ofCryptosporidiumin several stages of development within the patient's small intestine. The organism was resistant to all antibiotics used, and the patient died in the 5th month of his illness, despite receiving parenteral alimentation and undergoing thymus trans-plantation. His clinical course was similar to that of other patients with fatal immune deficiencies and cryptosporidiosis. Autopsy demonstrated organisms attacking the epithelium of the small intestine, pancreatic duct, and bronchioles. Multiple organ involvement in this patient negates the view thatCryptosporidiumis site-specific for a given host or is alimentary-canal-specific in humans.