The introduction of drug resistance testing during clinical care of patients infected with the human immunodeficiency virus (HIV) is still a matter for discussion. Thirty-seven HIV-infected subjects who had positive plasma viraemia despite undergoing antiretroviral therapy were tested for the presence of genotypic mutations linked to resistance to nucleoside analogues. Thirty (81.1%) individuals showed one or more mutations conferring drug resistance. Eight harboured mutants at codons 215 and 41, which confer resistance to zidovudine and are associated with the worst prognosis. One individual carried the codon 69-SSS insertion, which confers multidrug resistance. Seven (18.9%) subjects did not show any mutation, but all of them failed to adhere to the treatment regimen and/or experienced diarrhea that likely caused malabsorption of the drugs. Clinicians referred to the resistance profile when they adjusted or prescribed subsequent combination regimens for their patients.