Protozoans of the genus Cryptosporidium may cause serious diarrhoeal illness in immunocompromised hosts and especially in HIV-infected patients. In this study we have evaluated the frequency of Cryptosporidium in stools of 51 HIV-infected patients with diarrhoea. Laboratory diagnosis of cryptosporidiosis was performed of faecal samples concentrated by a formalin-ether sedimentation technique and stained by a modified cold Ziehl-Neelsen method. Results demonstrated that 17 (33.3%) of these patients were infected with Cryptosporidium. Moreover, Cryptosporidium infection was the first clinical marker of AIDS in 7 cases. Our data show that the prevalence of this parasitosis in HIV-infected people seems to be higher in our region (Apulia, South Italy), compared to other areas of the world.