Infertile men (excluding azoospermia) were divided into three groups to test methylcobalamin (CH3-B12) + clomiphene citrate (Clomid) treatment. Group M: CH3-B12 (1,500 micrograms/day, daily for 8 to 60 weeks), Group C: Clomid (25 mg/day for 25 days followed by a 5-day rest period, for 8 to 48 weeks), and Group MC: CH3-B12 + Clomid (CH3-B12: 1,500 micrograms/day, daily and Clomid: 25 mg/day for 25 days followed by a 5-day rest period, for 12 to 24 weeks semen analysis was conducted before and after these administrations. Judging by our criteria, 60.0% (24/40) of the cases in Group M, 71.4%(25/35) in Group C, and 66.7% (8/12) in Group MC were improved. However, in patients whose sperm concentrations were over 10 X 10(6)/ml, 61.9% (13/21) of the cases in Group M, 69.2% (9/13) in Group C, and 80.0% (4/5) in Group MC were improved. Therefore, CH3-B12 + Clomid treatment was successful for patients whose sperm concentrations was over 10 X 10(6)/ml.