Trimethoprim-sulfamethoxazole (TMP-SMZ) and pentamidine are both licensed for the treatment of Pneumocystis carinii pneumonia (PCP). However, their use is associated with various adverse side effects. In this prospective study, 26 AIDS patients with 32 episodes of PCP were treated with pentamidine (4 mg/kg/d). Each patient was treated for 12 to 21 days, depending on the rapidity of onset of the clinical response. During the 32 PCP episodes, hypoglycemia occurred in 16 instances, azotemia in 12, liver toxicity in 10, and leukopenia in 8. The occurrence of thrombopenia, leukopenia, and liver toxicity was not related to age, pentamidine levels, or other complications. However, patients who had hypoglycemia during pentamidine treatment had higher serum pentamidine levels than patients who did not have hypoglycemia (107 +/- 40 versus 70 +/- 26 ng/ml, p less than 0.004). In addition, we observed that patients with azotemia showed higher pentamidine levels during treatment (120 +/- 35 versus 64 +/- 22 ng/ml, p less than 0.001). In fact, 100% (11/11) of patients with serum pentamidine concentration greater than 100 ng/ml had fasting hypoglycemia and/or azotemia, while 33% (7/21) of those with pentamidine levels less than 100 ng/ml had these side effects (p less than 0.001). The relative risk of these complications with pentamidine levels greater than 100 ng/ml was 3 (95% confidence interval, 1.6 to 5.5). Fine-tuning the dose of pentamidine may eventually prove useful to avoid toxicity and optimize therapy.