Characteristics of afebrile infection episodes among adult patients with acute leukemia are here described. Afebrile episodes represented 14.3% of all infections. They significantly differed from the febrile episodes because: 1) they prevailed among patients with granulocyte count greater than 500/mm3 (p < 0.001); 2) they often involved patients in complete remission (p < 0.0002); 3) they affected more frequently the kidney and urinary tract than febrile infections (p = 0.0005) and 4) they lacked lung involvement (p < 0.01). The rate of documented infections by cultures, cytology or serological tests was not different between both infection types. Observed mortality during afebrile episodes was threefold less than febrile infections; this difference, however, did not reach statistical significance. In conclusion, afebrile infection in acute leukemia is a distinct clinical entity, unlike febrile infection.