The purpose of this study was to evaluate the usefulness of apparent diffusion coefficient (ADC) values using diffusion-weighted magnetic resonance imaging (DWI) in patients with ovarian cystic tumors. During past 12 months, we studied 30 patients who were clinically suspected of ovarian cystic tumors and underwent DWI using 1.5T MR unit. Eight patients with small cystic ovarian lesion less than 3cm in diameter and no sufficient DWI were excluded in calculating ADC value. Twenty-six cystic ovarian lesions of 22 patients were classified as four groups; ovarian cysts, cystadenomas, other benign tumors, and malignant tumors. DWI was obtained using the single-shot spin echo planar imaging and two gradient steps (b values of 0, 800 sec/mm2). The ADC values were measured using regions-of-interest (ROI) placed cystic components of DWI in the same section as T2-weighted image and kept away from septation and solid component. Mean ADC values were 0.196+/-0.105x10(-3) mm2/sec in ovarian cysts, 1.312+/-1.064x10(-3) mm2/sec in cystadenomas, 0.274+/-0.124x10(-3) mm2/sec in other benign tumors, and 1.011+/-0.080x10(-3) mm2/sec in malignant tumors. There were stastically significant differences in ADC values between cystadenomas and ovarian cysts or other benign tumor, and between malignant tumors and ovarian cysts or other benign tumors. There were no statistically significant differences in ADC values between ovarian cysts and other benign tumors, and between cystadenomas and malignant tumors (p>0.05). In conclusion, calculated ADC values using DWI would be helpful to differential diagnosis of cystic ovarian tumors.