Mutations in the human-derived Pneumocystis carinii dihydropteroate synthase (DHPS) gene have been reported with increasing frequency and have been linked to prior sulfa prophylaxis and possible emergence of sulfa resistance. This study was done to examine the prevalence and clinical significance of P. carinii DHPS mutations in Italian patients. A previously described single-strand conformation polymorphism technique was used to identify P. carinii DHPS mutations in 107 patients with acquired immunodeficiency syndrome. Overall prevalence (8%) was low compared with that in other reports. Mutations were observed in 19% (6/31) of patients exposed to sulfa prophylaxis, compared with 4% (3/76) of patients not exposed to sulfa prophylaxis (P=.017). No significant association was observed between the presence of DHPS mutations and mortality, CD4 cell count, or demographic factors. The study confirms the association between DHPS mutations and prior sulfa prophylaxis and shows that the prevalence of DHPS mutations in an Italian patient population is lower than that in other populations.