A double blind controlled study of bromocriptine treatment of oligospermia was carried out. Out of fifty-one men who originally volunteered to the study there were forty who took the drug for 12 weeks as requested. All the partners of these men had failed to conceive, and in each case the pretreatment sperm count had been below 40 million/ml on two or several occasions. The pretreatment serum prolactin concentrations were similar in patients given bromocriptine (N = 20) and placebo (N = 20). There were three men in either group whose pretreatment serum prolactin concentration was in excess of 30 micrograms/l, the highest value being 96 micrograms/l. While bromocriptine effectively decreased the serum prolactin concentration, it had no significant effect over placebo on sperm volume, motility and morphology. In the bromocriptine group, sperm count increased to or above 40 million/ml in five out of twenty men, while in the placebo group this occurred in nine out of twenty patients. The plasma testosterone and dihydrotestosterone levels increased slightly during treatment in both groups, but no significant difference was observed between bromocriptine and placebo treated patients. One wife of a bromocriptine-treated man and two wives of placebo-treated men became pregnant during treatment. In this study bromocriptine was no more effective than placebo in the treatment of oligospermia.