Therapeutic drug monitoring is essential in HIV-patients undergoing highly active antiretroviral therapy (HAART). Saquinavir (SQV) is used alone or in combination with ritonavir (RTV) or nelfinavir (NLF), respectively, in the context of the HAART drug regimen. The achievable SQV concentration range in clinical practice remains to be elucidated. A non-randomized prospecitve clinical trial 19 patients (group I) receiving SQV (1x600 mg/d Invirase or Fortovase), 29 patients (group II) receiving SQV (2x600 mg/d Fortovase) plus RTV (2x400 mg/d Norvir), and 21 patients (group III) receiving SQV (2x600 mg/d Fortovase) plus NLF (2x750 mg/d Viracept) was conducted to determine SQV plasma concentrations. SQV levels were determined as trough levels during routine outpatient visits. Analysis was performed by HPLC with UV detection. The lowest SQV plasma levels were found in group I (95% CI 89-177 ng/ml). Significantly higher SQV levels were found in group III (combination with NLF) ranging from 242 to 398 ng/ml (95% CI) and in group II (combination with RTV) ranging from 1354 to 1747 ng/ml (95% CI). The IC 50% of 54 ng/ml was not reached in at least one sample during the study (mean duration of study 16+/-10 months) in 14/19 patients of group I, 9/29 patients in group II and 13/21 patients in group III, respectively. A positive correlation between patient compliance, defined by SQV levels in the 95% CI of the used combination, and the HIV RNA plasma level was found. The presented data confirm that therapy with SQV alone may not be effective, since trough levels are near the lower limit of antiretroviral efficacy. Although the combination of SQV with NLF results in higher SQV plasma concentrations in a bid regimen, in more than 60% of the patients SQV concentrations below IC 50 level were detected during the twelve-months study period. The combination of SQV with RTV yields the highest SQV-trough levels. SQV concentrations below the IC 50 were seen in only 31% of patients with the SQV/RTV combination. In conclusion, therapeutic drug monitoring allows an efficient surveillance of patients compliance. In addition, therapeutic drug monitoring represents a valuable tool for management of HAART in patients receiving a complex comedication or suffering from advanced liver disease.