An HPLC method previously described for the assay of amprenavir (APV), ritonavir (RTV), indinavir (IDV), saquinavir (SQV), nelfinavir (NFV), lopinavir (LPV), atazanavir (ATV), nevirapine (NVP) and efavirenz (EFV) can be also conveniently applied, with minor gradient program adjustment, for the determination of the novel non-peptidic HIV protease inhibitor tipranavir (TPV) in human plasma, by off-line solid-phase extraction (SPE) followed by HPLC coupled with UV-diode array detection (DAD). After viral inactivation by heat, the plasma is diluted with phosphate buffer (pH 7), and subjected to a SPE on a C18 cartridge. Matrix components are eliminated with a solution of 0.1% H3PO4 solution neutralised to pH 7, and TPV is eluted with MeOH. The resulting eluate is evaporated and reconstituted in 100 microl MeOH/H2O 50/50. A 40 microl volume is injected onto a Nucleosil C18 AB column and TPV is analysed by UV detection at 201 nm using a gradient elution program constituted of MeCN and phosphate buffer adjusted to pH 5.12 and containing 0.02% sodium heptanesulfonate. The calibration curves are linear up to 75 microg/ml, with a lower limit of quantification of 0.125 microg/ml. The mean absolute recovery of TPV is 77.1+/-4.0%. The method is precise with mean inter-day coefficient of variations (CVs) within 2.2-3.4%, and accurate (range of inter-day deviations from 0.7 to 1.2%). The method has been validated and is currently applied to the monitoring of TPV plasma levels in HIV patients.