This study was performed to examine the effectiveness of steroid therapy in subfertile men with antisperm antibodies and infertility lasting > 1 year, to predict those who would respond positively, and to evaluate the effect of the therapy on semen parameters and antisperm antibodies. The patients included 48 subfertile couples in whom the male partner had > or = 20% motile spermatozoa with bound antibodies of immunoglobulin (Ig)G, IgA or a combination of both, and were treated with prednisolone, 40 mg a day, for the first 10 days, then 5 mg on days 11 and 12 of the partner's cycle for 9 months. Twelve couples became pregnant; a cumulative conception rate of 30.2% was achieved at 9 months. The pregnant group started with significantly higher concentrations of IgG (tail) and grade I motility (P = 0.03 and P = 0.02 respectively). Multi-covariate discrete logistic regression analysis on the initial screening semen samples predicted a higher chance of conception for those with high levels of IgG (tail) (P = 0.006, sensitivity = 33%, specificity = 93%, correct = 75%, false positive = 33% and false negative = 24%). In the pregnant group, prednisolone caused a significant increase in grade I motility (P = 0.03). In the non-pregnant group, there was a significant increase in grade I motility (P = 0.0002), amplitude of lateral head displacement (P = 0.03), curvilinear velocity (P = 0.02) and decrease in grade IV motility (P = 0.03) following prednisolone treatment. In both groups there was suppression of the total antisperm antibody concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)