Background: There are few data on the safety and antiviral activity of tenofovir disoproxil fumarate (TDF) in HIV-1 infected adolescents. Methods: A randomized, double-blinded, placebo-controlled study was conducted. Ninety adolescents (12 to <18 years) who were viremic while receiving antiretroviral treatment were randomized to receive TDF 300 mg (mean, 216.8 mg/m) or placebo in combination with an optimized background regimen (OBR) for 48 weeks. The primary efficacy endpoint was time-weighted average change in plasma HIV-1 RNA from baseline at week 24 Results: Eighty-seven subjects (45 TDF, 42 placebo) received the study drug. Through week 24, the median time-weighted average change in plasma HIV-1 RNA was not different between the TDF and placebo groups (-1.6 versus-1.6 log10copies/mL, P = 0.55). The percentages of subjects who achieved HIV-1 RNA <400 copies/mL were similar at week 24 (40.9 versus 41.5%). One fourth of subjects in the TDF and placebo groups (24.4 versus 28.6%) had at least 3 active agents in the OBR. Many subjects in both groups had baseline genotypic resistance to TDF (48.9 versus 33.3%). TDF was generally safe and well tolerated. There were no statistically significant differences in changes of renal function and bone mineral density between the 2 groups. Conclusion: This study of TDF in combination with an OBR in antiretroviral- experienced adolescents did not meet its primary or secondary efficacy endpoints. The effectiveness of the OBR and baseline genotypic resistance to TDF in both groups may have confounded the efficacy findings. No clinically relevant TDF-related renal or bone toxicities were observed in this adolescent population. 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