Endometrial curettings from 170 patients with all grades of endometrial hyperplasia, who did not undergo a hysterectomy for at least 1 year were evaluated in order to correlate the histopathologic features with behavior. Follow-up ranged from 1 to 26.7 years (mean, 13.4 years). Cytologic and architectural alterations were analyzed separately in order to assess their respective roles in predicting the likelihood of progression to carcinoma. Classification of proliferative lesions based solely on the presence of cytologic atypia revealed that atypia was a discriminant factor. Proliferations lacking cytologic atypia were designated hyperplasia and those displaying atypia were designated atypical hyperplasia. Only 2 (1.6%) of 122 patients with hyperplasia progressed to carcinoma compared with 11 (23%) of women with atypical hyperplasia (P = 0.001). Subclassification of the two forms of hyperplasia (those with cytologic atypia and those without) was performed using the degree of architectural abnormalities. Hyperplasia and atypical hyperplasia displaying marked glandular complexity and crowding producing a back-to-back appearance were designated complex hyperplasia (CH) and complex atypical hyperplasia (CAH), respectively. Hyperplasia and atypical hyperplasia with lesser degrees of glandular complexity and crowding were designated simple hyperplasia (SH) and simple atypical hyperplasia (SAH), respectively. Progression to carcinoma occurred in 1 (1%) of 93 patients with SH, in 1 (3%) of 29 patients with CH, in 1 (8%) of the patients with SAH, and in 10 (29%) of the patients with CAH. The differences between the four subgroups suggest a trend but are not statistically significant. The findings in this study provide a rationale for classifying noninvasive endometrial proliferations primarily on the basis of cytologic atypia since this is the most useful criterion in predicting the likelihood of progression to carcinoma. In addition, the presence of concommitant architectural alterations appears to identify a particularly high-risk subgroup.