DNA ploidy, mitotic rate (per 10 high power fields), mitotic index (per 1000 tumour nuclei), Ki-67 labelling index and S phase fraction were measured in 23 uterine leiomyosarcomas and 10 tumours of uncertain malignant potential. Correlations were calculated by Spearmann rank correlation. Univariate survival analysis was performed by log rank analysis and multivariate analysis performed by the Cox linear regression method. Ki-67 index and S phase fraction were significantly higher in leiomyosarcomas than in tumours of uncertain malignant potential. There was significant correlation between mitotic rate, mitotic index, Ki-67 index and S phase fraction in cases of leiomyosarcoma. Fifteen of 22 leiomyosarcomas and one of 10 tumours of uncertain malignant potential were DNA aneuploid. On univariate analysis of all the smooth muscle tumours, DNA ploidy, presence of significant nuclear atypia and presence of coagulative tumour cell necrosis were associated with outcome. Only DNA ploidy was associated with outcome in the group of leiomyosarcomas. On multivariate analysis of all of the smooth muscle tumours, DNA ploidy, age and grade of atypia were independently associated with outcome. No single factor was independently predictive of outcome in the group of leiomyosarcomas. Alternative indices of cell proliferation correlate with mitotic rate in uterine leiomyosarcoma and do not provide additional useful prognostic information. DNA ploidy, age and grade of atypia are independently associated with outcome in uterine smooth muscle tumours and measurement of DNA ploidy may be useful in identification of cases with an adverse prognosis.