In a prospective study the authors compared the clinical, radiological, biochemical and diagnostic features of Pneumocystis carinii pneumonia (PCP) in HIV-infected patients who were or were not receiving a specific prophylactic treatment. The study included 386 patients with suspected PCP, 201 of whom were under specific prophylactic therapy. Induced expectoration and/or bronchoalveolar lavage provided a diagnosis of PCP in 89 patients, 21 of whom were under specific prophylaxis. Apart from a number of circulating CD4 lymphocytes that was significantly lower in the patients under prophylaxis, there was no significant difference between the two groups. Thus, being under specific prophylaxis should have no effect on the diagnosis of PCP in HIV-infected patients.