OBJECTIVE To assess the likelihood of histologic persistence/progression of complex hyperplasia and atypical hyperplasia among women treated with progestin compared with those not treated, with attention to type, dose, and duration. METHODS This was a cohort study at an integrated health plan of women, ages 18-85 years, with complex or atypical hyperplasia on independent pathology review with a second endometrial specimen in the 2-6 months after the index diagnosis. Progestin therapy between index diagnosis and follow-up biopsy was determined from the pharmacy database. Medical record abstraction was performed. Relative risks (RRs), adjusted for age and body mass index, were calculated. RESULTS Among 185 women, average age 55.9 years, follow-up 16.1 weeks, 115 had complex and 70 had atypical hyperplasia. Among women with complex hyperplasia, 28.4% of those treated with progestin and 30.0% of those not treated had persistence/progression (RR 1.20, 95% confidence interval [CI] 0.53-2.72). Among women with atypical hyperplasia, 26.9% of those treated with progestin and 66.7% of those not treated had persistence/progression (RR 0.39, 95% CI 0.21-0.70); there was a suggestion that use of at least a medium dose, or a duration of at least 3 months, was associated with a particularly low probability of persistence/progression. CONCLUSIONS Although progestin treatment of women with atypical hyperplasia was associated with a substantial increase in the likelihood of regression of the lesion during the ensuing 2-6 months, persistence/progression was nonetheless present in more than one quarter of treated women. Regression of complex hyperplasia without atypia was common whether progestin had or had not been used.