Antisperm antibodies have been measured in serum and in seminal plasma in 130 males before and after vasectomy reversal, and the occurrence of pregnancy has been analyzed in those partners who were trying to produce a pregnancy. All patients have been followed for at least one year. Sperm-agglutinating antibodies were found in the serum of 79% of patients; seminal plasma antibodies were present in only 9.5% before reversal, and this rose to 29.5% afterwards. Overall, pregnancies occurred in the partners of 44.6% of those men who were trying to produce children. Production of pregnancy was significantly less likely when the preoperative serum antisperm antibody titer was 512 or more, but no decrease in fertility was seen with titers below this. Similar numbers of pregnancies were produced by patients with or without seminal plasma antibodies in titers of up to 16; there are too few patients with titers above this level to permit further analysis. A randomized controlled trial of perioperative steroids showed that they produced no benefit. It appears that the antisperm antibodies associated with vasectomy reversal may differ fundamentally from those occurring in naturally subfertile males.