The interindividual variability of tenofovir pharmacokinetics in HIV+ patients is quite large, but the sources of variability are incompletely understood. Intraindividual variability has not been characterized, although it may have an impact on efficacy and therapeutic drug monitoring. The aims of the study were to estimate intraindividual variability of tenofovir clearance and to assess interactions with associated antiviral drugs. Tenofovir concentrations (median 2; range, 1-5) were measured in 175 patients during several dosing intervals. Covariates and dosing regimen of associated antiretroviral drugs were recorded prospectively. The data were analyzed by a population approach. The final model was a 2-compartment model with first-order absorption rate. The elimination clearance was found to be related to the ratio of body weight to serum creatinine. Among the 15 drugs coadministered, no interaction on tenofovir kinetics was significant. The global variability of CL/F, after accounting for variability to variation of body weight and serum creatinine, was about 50%, with 20% due to interindividual variability and 30% due to interoccasion variability. In a few patients, clearance (and AUC) could vary by a factor of 2 between occasions. The interoccasion variability was not related to the delay between occasions. In the context of drug monitoring, for a given patient, the dose should not be adapted unless the variation of concentration between 2 occasions is large, or the 24-hour trough concentration at steady state is lower than 12 microg/L.