The influence of nevirapine or efavirenz co-administration on ritonavir-boosted amprenavir pharmacokinetics was investigated in HIV-infected patients with a population pharmacokinetic approach. The analysis was performed with a population of 61 patients treated with fosamprenavir/ritonavir (700 mg/100 mg twice daily) combined with nucleoside/nucleotide reverse transcriptase inhibitors +/- enfuvirtide and no other antiretroviral drugs (group A, n = 46) or nevirapine (group B, n = 10) or efavirenz (group C, n = 5). No significant increase in amprenavir clearance [mean +/- standard deviation: 22.49 +/- 10.32 (group A) vs. 21.57 +/- 9.62 (group B) vs. 20.15 +/- 5.18 (group C) L/h] and no significant decrease in trough amprenavir plasma concentrations [1.75 +/- 0.95 (group A) vs. 1.82 +/- 0.72 (group B) vs. 1.55 +/- 0.66 (group C) mg/L] were found in groups B and C in comparison with group A, although nevirapine and efavirenz are inductors of protease inhibitors metabolism. These results suggest that fosamprenavir/ritonavir should be used at standard doses of 700 mg/100 mg twice daily when combined with efavirenz or nevirapine.