Introduction - Pneumocystis pneumonia (PCP) is one of the leading opportunistic fungal infections. To date, properly administered cotrimoxazole prophylaxis remains effective. However, genetic markers of potential resistance to sulfonamides—the primary drugs used in chemoprophylaxis—have been reported: mutations in the gene encoding dihydrofolate synthase (DHPS). Method - The network, comprising 14 hospitals in the Paris region, collects data on the epidemiology of Pneumocystis carinii pneumonia and enables genotyping of strains; hospital laboratories report clinically and biologically confirmed cases. DHPS mutations are detected by PCR-RFLP. Results - From January 1, 2003, to December 31, 2005, 471 cases were reported. 64% of patients were HIV-positive. Men accounted for 69% of cases, and the median age was 46 years. Among HIV-positive patients, PCP revealed the viral infection in 44% of cases. Among known HIV-positive patients, no prophylaxis was prescribed in 74% of cases; the 23 reported cases of cotrimoxazole prophylaxis were associated with non-adherence or intolerance; these were second episodes in 16% of cases. Among HIV-negative patients, corticosteroid therapy was prescribed in 60% of cases and chemotherapy in 49% of cases; 91% did not receive prophylaxis. DHPS mutations were detected in 47 of the 183 samples studied, with no association with mortality or prior prophylaxis. (R.A.)