27% of 89 young, non-ill, homosexually active men in Los Angeles had a Leu-3(OKT4)/Leu-2(OKT8) ratio less than or equal to 0 . 8. The low ratio was due to a significantly raised mean number of Leu-2 cells, whereas individuals with acquired immune deficiency syndrome had a decrease in both Leu-3 and Leu-2 cells. The term "acquired immune augmentation" may be appropriate for those with a low ratio due to raised numbers of Leu-2 cells. Those practising passive (receptive) anal intercourse had a significantly higher mean number of Leu-2 cells than did those practising only active (insertive) anal intercourse or no anal intercourse. The results of this study suggest that two distinct conditions occur in homosexually active men. Whether acquired immune augmentation is a precursor of the rarer acquired immune deficiency syndrome or is an unrelated disorder needs to be determined. In this study passive anal intercourse was associated with acquired immune augmentation.