With the use of semi-thin sections of 88 testicular biopsies from 44 patients, we were able to separate a relatively uniform group of patients with idiopathic left varicocele and infertility into 4 groups with different pathophysiological conditions. Group 1 patients had atrophy of the Leydig cells, decreased Leydig cell ratio, low plasma testosterone, and normal luteinizing and follicle-stimulating hormone levels. Surgery resulted in a significant improvement in sperm count. Group 2 patients had undergone attempted repair of Leydig cells, and had a normal Leydig cell ratio, and normal testosterone, luteinizing hormone and follicle-stimulating hormone plasma values. Group 3 patients demonstrated hyperplasia and an increased Leydig cell ratio, high luteinizing and follicle-stimulating hormone values, and relatively high testosterone values. Group 4 patients had an increased Leydig cell ratio but severe atrophy of the Leydig cells and tubuli, indicating burned out testes with high luteinizing and follicle-stimulating hormone but low testosterone levels. A successful operation failed to increase the sperm count in the latter groups. The technique used should allow better patient selection for medical treatment, as well as lead to a better understanding of the etiopathogenesis of infertility in varicocele patients. Therefore, the histological technique has an important role in the evaluation of patients suffering from infertility in general.