OBJECTIVE To determine whether an association existed between lopinavir (LPV) plasma concentrations and changes in lipid levels. METHODS A prospective, nonrandomized study. METHODS HIV-infected subjects with virologic failure on protease inhibitor-containing regimens. Twenty-two consecutive patients were enrolled, 19 completed 24 weeks of treatment, and 16 completed the full 48-week study period. INTERVENTION Patients were treated with LPV/ritonavir (LPV/r) in combination with other antiretroviral agents. Subjects were evaluated at baseline and weeks 4, 8, 12, 24, 36, and 48. LPV trough plasma concentrations and lipid levels were measured. RESULTS LPV trough concentrations were higher in patients experiencing grade 3 or higher lipid elevations (mean [SD]: 9.71 microg/mL (5.62) vs. 6.09 microg/mL (3.83); P = 0.002) and in those developing grade 2 or higher hypercholesterolemia (mean [SD]: 8.48 microg/mL [4.64] vs. 5.71 microg/mL [3.94]; P = 0.003). All patients developing grade 2 or higher cholesterol elevation had an LPV trough concentration at week 4 greater than 8 microg/mL. Significant positive correlations were found between LPV trough concentrations and changes in triglyceride and cholesterol levels. CONCLUSIONS In patients receiving salvage therapy with LPV/r, there is an association between LPV plasma concentrations and lipid changes. Patients achieving higher LPV trough concentrations may be at greater risk of experiencing dyslipidemia. Further investigations are warranted to support a direct cause and effect relationship.