The opportunistic fungal pathogen Pneumocystis carinii sp. f. hominis is a frequent cause of pneumonia in the immunocompromised host. Analysis of genetic variation among isolates of P. carinii sp. f. hominis from 12 human immunodeficiency virus (HIV)-infected persons with single and multiple episodes of P. carinii pneumonia was undertaken at the internal transcribed spacer (ITS) regions of the nuclear rRNA operon. In samples from 24 episodes of pneumonia, 10 different types of P. carinii sp. f. hominis were identified. More than 1 sequence type was observed in g samples, indicating that mixed infection with different types of P. carinii sp. f. hominis is not uncommon. In 4 of 7 patients with recurrent episodes of pneumonia, the sequence types observed at the second episode were different from those of the first, suggesting the occurrence of both reactivation of a previously acquired infection and reinfection from an exogenous source.