Aim: To study plasma fatty acid composition in human immunodeficiency virus-infected children treated with protease inhibitors and its relation with other components of the metabolic syndrome observed after this therapy. Design: Cross-sectional study from collected clinical database. Subjects: 17 children with HIV infection treated with protease inhibitors. Nine patients received ritonavir (20-30 mg/kg/d)-and the remaining eight received nelfinavir (60-90 mg/kg/d). Duration of protease inhibitors treatment was 711 ±208 d. As controls, we used 112 matched blood samples from apparently healthy children admitted for minor surgical procedures. Methods: Plasma fatty acids were determined using a Hewlett Packard GC 5890 gas chromatograph. Results: Plasma levels of cholesterol and triglycerides and insulin-like growth factor 1 (IGF-1) tended to be high in protease inhibitor-treated patients. Plasma content of ω6 long-chain polyunsaturated fatty acids and, in particular, of the highly unsaturated 22:4ω6 and 22:5ω6, was significantly increased. Also, infected children had increased Δ6 and Δ4 desaturase activities and decreased Δ5 desaturase activity. Significant correlations were present between plasma IGF-1 level and plasma triglycerides, plasminogen activator inhibitor-1 activity and Δ6 desaturase activity. Conclusion: HIV-infected, protease inhibitor-treated children exhibit a metabolic syndrome which is associated with signifi-cant changes in plasma fatty acid composition. These changes are similar to those observed in situations of insulin resistance and are linked to variations in plasma IGF-1 concentration. Aim: To study plasma fatty acid composition in human immunodeficiency virus-infected children treated with protease inhibitors and its relation with other components of the metabolic syndrome observed after this therapy. Design: Cross-sectional study from collected clinical database. Subjects: 17 children with Nine patients received ritonavir (20-30 mg / kg / d) -and the remaining eight received nelfinavir (60-90 mg / kg / d). Duration of protease inhibitors treatment was 711 ± 208 d. Methods: Plasma fatty acids were determined using a Hewlett Packard GC 5890 gas chromatograph. Results: Plasma levels of cholesterol and triglycerides and insulin-like growth factor 1 (IGF-1) tended to be high in protease inhibitor-treated patients. Plasma content of ω6 long-chain polyunsaturated fatty acids and, in particular, of the highly unsatu rated 22: 4ω6 and 22: 5ω6, was significantly increased. Also, infected children had increased Δ6 and Δ4 desaturase activities and decreased Δ5 desaturase activity. Significant correlations were present between plasma IGF-1 level and plasma triglycerides, plasminogen activator inhibitor-1 activity and Δ6 desaturase activity. Conclusion: HIV-infected, protease inhibitor-treated children exhibit a metabolic syndrome which is associated with signifi-cant changes in plasma fatty acid composition. These changes in similar to those observed in situations of insulin resistance and are linked to variations in plasma IGF-1 concentration.