Purpose:To evaluate the value of diffusion-weighted imaging (DWI) in distinguishing between benign and malignant breast lesions. Materials and Methods:Fifty-two female subjects (mean age ϭ 58 years, age range ϭ 25-75 years) with histopathologically proven breast lesions underwent DWI of the breasts with a single-shot echo-planar imaging (EPI) sequence using large b values. The computed mean apparent diffusion coefficients (ADCs) of the breast lesions and cell density were then correlated. Results:The ADCs varied substantially between benign breast lesions ((1.57 Ϯ 0.23) ϫ 10 -3 mm 2 /second) and malignant breast lesions ((0.97 Ϯ 0.20) ϫ 10 -3 mm 2 /second). In addition, the mean ADCs of the breast lesions correlated well with tumor cellularity (P Ͻ 0.01, r ϭ -0.542). Conclusion:The ADC would be an effective parameter in distinguishing between malignant and benign breast lesions. Further, tumor cellularity has a significant influence on the ADCs obtained in both benign and malignant breast tumors.