We examined samples from 98 naïve HIV-1 positive patients with the seroconversion period between 1984 and 1997, 64 of whom with a diagnosed primary infection. We observed a progressive increase in the percentage of patients harboring mutations associated with zidovudine resistance, starting from 8% during the period 1987-1994 to 20% and 36% in patients with HIV infection diagnosed during 1996 and 1997 respectively. The small number of patients analyzed in groups 3 and 4 is an important limitation to establish the real increase in the prevalence of mutations we observed during the past two years. However, it is important to underline the trend and, in our opinion, further studies to better define the relevance of such phenomenon in the clinical practice must be performed.