A study of 34 full-thickness Stein-Leventhal ovarian wedges and 30 age-matched control ovaries allowed comparison of the entire ovarian cross-sections and quantification of their features. As compared with controis, Stein-Leventhal ovaries showed on average (i) double the cross-sectional area, (ii) the same number of primordial follicles, (iii) double the number of ripening and subsequent atretic follicles from the earliest stages, (iv) a tunica increased by 50 per cent and more collagenized, (v) cortical stromal thickness increased by a third, (vi) subcortical stroma, whether deep cortical or medullary in site, increased five times, (vii) ovarian hilus cell nests four times as frequently. The increased subcortical stroma was derived partly from the regressive conversion into stroma of the over-numerous older follicles, so augmenting steadily with duration, and partly from a concurrent stromal hyperplasia. Stromal smooth muscle and lutein cell nests were each found in four-fifths of cases. So-called “hyperthecosis,” in which such nests are combined with marked stromal increase, is arguably just a late stage of Stein-Leventhal morphology. The whole picture may result directly or indirectly from the raised LH output, although androgens possibly promote early follicle ripening.