Endocrine studies in men with pubertal macromastia (PM) have failed to reveal a hormonal abnormality to account for the disorder. As a result, it has been hypothesized that this form of gynecomastia may be a manifestation of a target organ abnormality. Because the syndromes of apparent androgen resistance, such as testicular feminization and Reifenstein's Syndrome, are associated with gynaecomastia, we examined skin fibroblasts cultured from men with pubertal macromastia for androgen receptor defects. We studied 12 men with PM and confirmed that plasma concentrations of testosterone, oestradiol, gonadotrophins, and prolactin, were all within the normal range; findings identical to those of a similar series of patients previously reported. Androgen receptor content (R0) and binding affinity (Kd) in cultured areolar and pubic skin fibroblasts were measured using a dispersed, whole cell assay. Nineteen areolar cell lines from the 12 patients with PM were compared with 4 areolar cell lines from three normal men and 9 areolar cell lines from nine normal women. There was no difference in the mean androgen receptor content (approximately 10 000 sites/cell) or binding affinity (approximately 1 nM) between the patients' fibroblasts and those of the normal subjects. Similarly, there were no differences in these parameters when pubic skin fibroblast androgen receptors were used for the comparison. We conclude that, although PM may yet be due to a defect in breast tissue sensitivity to androgen, the disorder cannot be explained by abnormalities in fibroblast androgen receptor number or affinity.