A total of (408) immunocompromised Saudi patients (<2 - >60 years) checked for Cryptosporidium infection showed 69.7% and 64.2% infection rates by Kinyoun's acid fast staining for oocysts and a monoclonal ELISA kit for C. parvum coproantigen detection respectively (P > 0.05). Microscopic examination of stained faecal smears was more sensitive than ELISA (P > 0.05) & showed 84.4% sensitivity, 81.7% specificity & 83.8% diagnostic accuracy. Infection rates of 47.6%, 78.2% & 82.3% were obtained by microscopic examination for children with chronic diarrhea and malnutrition, patients receiving immunosuppressive (I.S.) drugs for organ transplantation or cancers and patients with HIV and chronic diarrhoea respectively. Infection rates of 84% & 74.3% were in patients receiving I.S. drugs for organ transplantation and malignancy respectively. In all patients, the highest infection rate (84%) was among age group 16-40 years while the least (35.3%) was among infants <2 years (P<0.001). Infections in males (73.9%) was significantly (P < 0.05) higher than females (62.6%). This high prevalence rate revealed the first reported for cryptosporidiosis among immunocompromised Saudis, indicating the presence of infection source(s) (nosocomial), and thus, transmission in Saudi Arabia.