A prospective study of the frequency, timing, etiology and risk factors of infections in renal transplant recipients during the first year after transplantation was conducted in 50 consecutive patients. Neither prophylaxis with trimethoprim-sulfamethoxazole nor antiviral prophylaxis was administered. Two hundred twenty-eight episodes of infection were registered (4.5 per patient), 19 of which were severe. Forty-seven percent of all infectious episodes occurred during the first two months after transplantation. The more frequent infections were bacterial (64%), viral (22%) and fungal (11%). Escherichia coli was the most common agent isolated (n = 36), followed by cytomegalovirus (n = 32). Urinary tract infections were most common (n = 144), especially asymptomatic bacteriuria (n = 106). Surgical reintervention and the use of anti-lymphocytic globulins were associated with a higher frequency of severe infections (p < 0.05), and invasive candidiasis was associated with allograft loss (p < 0.03). Annual survival rates of patients and allografts were 100% and 94%, respectively. The frequency of mild infections was higher than that observed in other studies using bacterial or viral prophylaxis. Nevertheless, the number of severe infections and the survival rates of patients and allografts were similar to those reported in previous studies.