Background HIV is transmitted primarily through sexual intercourse, and the objective of this study was therefore to assess whether there is occult viral replication and resistance in genital secretions in patients on protease inhibitor (PI)-based second-line therapy. Methods HIV-infected adults taking ritonavir-boosted lopinavir with either two nucleoside reverse transcriptase inhibitors (NRTIs), raltegravir or as monotherapy for 96 weeks, were enrolled at seven clinical sites in Uganda. Viral load (VL) was measured in cervico-vaginal secretions or semen and in a corresponding plasma sample. Genotypic resistance was assessed in genital secretion samples and plasma samples. Results were compared between compartments and with the plasma resistance profile at first-line failure. Results Of the 111 participants enrolled (91 female, 20 male), 16 (14%) and 30 (27%) had VL >1,000 and >40 copies/ml, respectively, in plasma; 3 (3%) and 23 (21%) had VL >1,000 copies/ml and >40 copies/ml, respectively, in genital secretions. There was 74% agreement between plasma and genital secretion VL classification above/below 40 copies/ml threshold (kappa-statistic =0.29; P=0.001). RT mutations (both NRTI and non-nucleoside reverse transcriptase inhibitor) were detected in genital secretions in four patients (similar profile to corresponding plasma sample at first-line failure) and PI mutations were detected in two (one polymorphism with no impact on resistance; one with high-level PI resistance). Conclusions High level (>1,000 copies/ml) viral replication and development of new RT or PI resistance in the genital compartment were rare. The risks of transmission arising from resistance evolution in the genital compartment are likely to be low on PI-based second-line therapy.