In order to ascertain the reproducibility of the mitosis count in histologic tumor diagnosis, a reference set of 10 microscopic slides from smooth muscle tumors of the uterus was shown to six different pathologists, who were asked to record the number of mitotic figures per 10 high power fields in the most active region of each slide. The results, when tubulated, revealed considerable observer variation in this supposedly objective and quantitative diagnostic criterion. The diagnosis of benignity or malignancy, based on this criterion alone, were unamimous in only four of the 10 cases, whereas the diagnoses based on a constellation of histologic criteria were unanimous in all cases and correlated well with the subsequent clinical evolution of those cases with follow-up data. Possible reasons for the lack of reproducibility of the mitosis count are discussed. We conclude that a qualitative estimation of mitotic activity is helpful as one of the diagnostic and prognostic criteria in the assessment of uterine and other smooth muscle tumors, but that it cannot be used as the sole criterion for distinguishing benign from malignant tumors.