PURPOSE: To investigate the eligibility of diffusion-weighted imaging (DWI) for the evaluation of tumor cellularity in patients with soft-tissue sarcomas. MATERIALS AND METHODS: Thirty consecutive patients with a total of 31 histologically-proven soft-tissue sarcomas prospectively underwent magnetic resonance imaging (MRI) including DWI with echo-planar imaging (EPI) technique immediately before open biopsy (N = 1) or tumor resection (N = 30). Fourteen patients had no previous anticancer treatment, 16 had received neoadjuvant therapy. Tumor cellularity as determined from histological sections was compared with minimum apparent diffusion coefficient (ADC). RESULTS: Tumor cellularity correlated well with minimum ADC in a linear fashion, with a Pearson correlation coefficient of -0.88 (95% confidence interval [CI]: -0.75 to -0.96). This relationship was not influenced by prior anticancer treatment. There was only a tendency toward lower ADC in tumor with higher grading but no significant dependency (P = 0.08). CONCLUSION: DWI has proven useful for the assessment of tumor cellularity in soft-tissue sarcomas. In result, DWI may be used as a powerful noninvasive tool to monitor responses of cytotoxic treatment as reflected by changes in tumor cellularity.