To estimate the incidence of occult uterine sarcoma and leiomyosarcoma in hysterectomies for leiomyomas and the risk associated with their morcellation. We conducted a population-based cohort study. All uterine sarcomas from 2006-2013 in an integrated health care system were identified. Age- and race-specific incidences of occult uterine sarcoma were calculated. Kaplan-Meier survival analysis was performed. Crude and adjusted risk ratios of recurrence and death associated with morcellation at 1, 2, and 3 years were estimated using Poisson regression with inverse probability weighting. There were 125 hysterectomies with occult uterine sarcomas identified among 34,728 hysterectomies performed for leiomyomas. The incidence of occult uterine sarcoma and leiomyosarcoma was 1 of 278 or 3.60 (95% confidence interval [CI] 2.97-4.23) and 1 of 429 or 2.33 (95% CI 1.83-2.84) per 1,000 hysterectomies. For stage I leiomyosarcoma (n=111), eight (7.2%) were power and 27 (24.3%) nonpower-morcellated. The unadjusted 3-year probability of disease-free survival for no morcellation, power and nonpower morcellation was 0.54, 0.19, and 0.51, respectively (P=.15); overall survival was 0.64, 0.75, and 0.68, respectively (P=.97). None of the adjusted risk ratios for recurrence or death were significant except for death at 1 year for power and nonpower morcellation groups combined (6/33) compared with no morcellation (4/76) (5.12, 95% CI 1.33-19.76, P=.02). We had inadequate power to infer differences for all other comparisons including 3-year survival and power morcellation. Morcellation is associated with decreased early survival of women with occult leiomyosarcomas. We could not accurately assess associations between power morcellation and 3-year survival as a result of small numbers.