The purpose of this study is to investigate the utility of the apparent diffusion coefficient (ADC) in differentiating benign and malignant uterine smooth muscle tumors classified by signal intensity (SI) on T2-weighted imaging (T2WI) and diffusion-weighted imaging (DWI), and to determine the correlation between ADC and tumor cell density. This retrospective study reviewed 168 lesions in 134 cases with pathologically confirmed uterine smooth muscle tumors, including 6 leiomyosarcomas and 3 smooth muscle tumors of uncertain malignant potential, and preoperative magnetic resonance imaging examinations performed between October 2009 and November 2012. T2WI and DWI were also performed for each subject. Tumors were then classified according to SI on T2WI and DWI relative to myometrial SI. The correlation between ADC and tumor cell density was also determined. In Group 1 (high on both T2WI/DWI), mean ADC was significantly lower for leiomyosarcoma (0.91 x 10(-3) mm(2)/s) than for leiomyoma (1.30 x 10(-3) mm(2)/s; p < 0.05) and mean cell density significantly higher for leiomyosarcoma (42.9%) than for leiomyoma (22.4%; p < 0.05). A strong negative correlation was seen between ADC and tumor cell density in Group 1 (Spearman, R = -0.72; p < 0.05). ADC may help to differentiate benign from malignant uterine smooth muscle tumors, particularly tumors with high SI on T2WI and DWI.