Sixteen cynomolgus macaques were intravenously infected with SHIV89.6P and exposed to Pneumocystis (Pc) via natural transmission and cohousing with other SIV+/Pc+ macaques. Eleven macaques became Pc-colonized, and 5 remained Pc-negative by 25 weeks post-SHIV infection and Pc-exposure, during which time serial blood and BAL fluid samples, as well as pulmonary function data, were collected. At 25 weeks post-SHIV infection, trimethoprim- sulfamethoxazole (TMP-SMX) treatment was initiated in 7 randomly-selected Pc-colonized animals and in all 5 Pc-negative monkeys. Four Pc-colonized animals were withheld from TMP-SMX treatment. Blood, BAL fluid and pulmonary function data collection was continued the remainder of the study duration, and at 72 weeks post-SHIV infection (47 weeks post TMP-SMX initiation), animals were sacrificed.