Approximately 20 thousand bovines of autochtonious and imported breeds, located in different regions of the Country were serologically tested by Immuno-diffusion (I.D.) in agar, using BLV GP60 antigens, for the purpose of determining the prevalence of infection by Bovine Leukemia Virus. Hematologic and histopathologic assays were made with the object of determining variations in hematologic values and the determination of stages of persistent lymphocytosis, and of studying the morphocytological changes that accompany malignant transformations of lymphoid elements, in order to make studies on the incidence of clear tumoral lesions in herds highly affected by Leukemia. From 16 thousand samples of serum, tested by the I.D., 6 thousand were positives (37,5%) pertaining to 150 herds distributed in all 20 states of the Country. The sera correspond to different breeds: dairy cattle, beef cattle, brahman, zebú hybrids, and buffalo. Hematologically, persistent lymphocytosis induced by the BLV was verified, by previously discarding other pathologic stages with transitory lymphocytosis. Factors which explain this high incidence and the spread of the Bovine Leukemia in Venezuela were detected.