Seminal abnormalities are commonly found during sulphasalazine treatment. Although these changes appear reversible after drug withdrawal this may result in colitis relapse. Animal studies suggest that 5-aminosalicylic acid, the active component of sulphasalazine, does not impair fertility. Sixteen patients with quiescent ulcerative colitis were studied. Each patient produced three samples of semen at weekly intervals. Of the 48 samples analysed 39.6% showed oligospermia, 41.7% showed an increased number of abnormal forms and 91.7% showed impaired motility. Nine patients substituted enteric coated mesalazine (5-aminosalicylic acid) for sulphasalazine for a minimum period of three months. During this time one patient developed a salmonella associated colitis relapse; the others remained well. Improvement in sperm count (p less than 0.02), motility (p less than 0.001) and morphology (p less than 0.02) occurred in all cases. To date, four successful pregnancies have resulted, three in couples complaining of long term infertility. Treatment with enteric-coated mesalazine allows the recovery of seminal abnormalities induced by sulphasalazine in patients with colitis.