A mathematical model of the interaction among CD4+ T-cells, HIV-1, and antiretroviral drugs was fitted to the viral load decline following initiation of combination therapy to estimate differences in the residual reproductive capacity of virus (R0) in the average patient in each group. Four regimens were studied: 12 patients on 5-drug nucleoside reverse transcriptase inhibitor (NRTIs), nonnucleoside reverse transcriptase inhibitor (NNRTIs) and protease inhibitor (PI)-containing combination therapy, 11 patients on PI-containing triple therapy, 10 patients on double NRTI therapy, and 10 patients on NNRTI- containing triple therapy. Model fits were used to estimate R0. The NNRTI-containing triple therapy and the 5-drug regimen blocked viral replication to the greatest extent (R0 = 0.85; 95% confidence interval [CI], 0.79-0.91; and 0.90, 95% CI, 0.82-0.98, respectively), with the former being significantly better than the PI-containing triple regimen (R0 = 0.98; 95% CI, 0.92-1.03; p = .007). Both the NNRTI-containing and the 5-drug regimen, as well as the PIcontaining triple therapy, were significantly better at blocking viral replication than the double NRTI therapy (RO = 1.04; 95% CI, 1.0-1.07). Measurement of viral load after approximately 7 days provided the most accurate measure of the degree of viral suppression induced by a given drug regimen.