OBJECTIVE To describe the evolution of resistance to zidovudine (ZDV), lamivudine (3TC), abacavir (ABC) and nelfinavir (NFV), 113 previously untreated children in the PENTA 5 trial had resistance assayed at baseline, rebound and/or 24, 48, 72 weeks (VIRCO: phenotyping and genotyping with 'Virtual Phenotype' interpretation). RESULTS At baseline, few reverse transcriptase mutations and no primary protease inhibitor mutations were observed. Time to detectable HIV-1 RNA with reduced phenotypic susceptibility to any drug was shortest in the ZDV+3TC arm (overall logrank P=0.02). Through a median follow-up of 55 weeks, at their last assessment 11 (28%), 16 (40%) and 13 (32%) children with detectable HIV-1 RNA and a resistance test available had mutations conferring resistance to none, one, or two or more trial drugs, respectively, according to the virtual phenotype. Reduced phenotypic susceptibility to ABC only occurred in the 3TC+ABC arm and required K65R and/or L74V in addition to M184V. NFV-resistant virus was selected slowly through D30N or L90M pathways, and selection of ZDV-resistant virus was rare. CONCLUSIONS Selection of 3TC-resistant virus was most frequent, followed by NFV and/or ABC; selection of ZDV-resistant virus was rare. Importantly, although in vitro, ABC selects for M184V as the first mutation, ABC did not select for M184V when combined with ZDV without 3TC. The most sustained HIV-1 RNA response was in the 3TC+ABC arm, but mutations conferring reduced susceptibility to 3TC and/or ABC evolved more frequently if virological failure occurred with 3TC+ABC than with ZDV+ABC.