A total of 95 clinically healthy and seronegative for Leishmania infantum dogs, residing an area highly endemic for canine leishmaniosis (CL) and living an outdoor life-style, were split into positive and negative groups, and then were randomly assigned to receive allopurinol (n=51; 20 mg/kg once daily), or placebo (n=44) for 1 week per month, from April to November. Forty per cent (38/95) of these dogs were not reexamined and retested at the end of the trial for reasons unrelated to CL. None of the remaining 57 dogs exhibited the symptomatic form of the disease at the end of the 1-year follow-up period. Of the 15 allopurinol-treated dogs that were non-infected (negative PCR and tissue smear microscopy) at the beginning of the trial, 6 (40% P=0.03) became PCR-positive, of which 3 became also seropositive, at the end of the observation period. In contrast, only 1 of 7 (14.3%) placebo-treated non-infected dogs became PCR positive at the same time point. Of the 19 allopurinol-treated dogs that were infected (PCR-positive) at the beginning of the trial, 18 (94.7%) remained PCR-positive and one (5.3%) seroconverted, at the end of the observation period. Of the 16 initially infected and placebo-treated dogs, 14 (87.5%) remained PCR positive, of which one (6.7%) also turned positive by tissue smear microscopy. Therefore, it is concluded that the use of allopurinol, at the daily dose of 20mg/kg, for 1 week per month, during the period of sandfly activity, does not prevent the infection of non-infected dogs by L. infantum, and, also, does not help in the elimination of the parasite from dogs with asymptomatic infections.