We examined the prevalence and the extent of prolongation of the PR and QRS intervals and their relation to anti-HIV treatments and other clinical characteristics in 970 HIV-infected patients, 749 treated with antiretroviral therapy and 221 untreated. Age, body mass index, heart rate, and treatment with β-blockers and HIV protease inhibitors (PIs) were independent predictors of increase in the duration of the PR interval. Male gender, Caucasian ethnicity, heart rate, duration of antiretroviral therapy, and use of PIs were independent predictors of an increase in the duration of the QRS interval. Users of HIV PIs had an adjusted QRS-interval duration that was 2.6 ms (95% confidence interval (CI) 1.4-3.9) longer than the interval in nonusers (P = 0.0004). The adjusted odds ratios of first-degree atrioventricular block (n = 54) and complete bundle branch block (n = 23) were 1.62 (95% CI 0.90-2.89; P = 0.10) and 2.71 (95% CI 1.10-7.13; P = 0.03), respectively, in patients taking PIs. These findings may have important clinical implications, particularly with respect to QRS prolongation in patients with myocardial ischemia or heart failure.