To know the parameter related to improvement of each semen characteristic (sperm density and percentage of progressive motile sperm) after high ligation of the left internal spermatic vein, 119 men from infertile couples were investigated. In each individual, the semen characteristic was considered to be different from the postoperative mean value if it was over or below 2 SD of the preoperative mean. Although no relationship was detected with the age of the patient, the duration of infertility or the preoperative semen characteristics, a definite relationship was found between the degree of varicocele and the improvement in each semen characteristic. For improvement of the sperm density, each of the volume and the score count by Johnsen's scale of the affected and unaffected testes and the serum hormone values (LH, FSH and testosterone) showed correlations. Meanwhile, for the improvement of the percentage of progressive motile sperm, correlations were found only with each of the testicular volume on the unaffected side and the serum testosterone level.