Tumor growth on T2-W MRI in 151 men with low-risk prostate cancer managed by active surveillance was related to tumor apparent diffusion coefficient (ADC). Volume increases greater than the 95% upper Limits of Agreement of reproducibility (~60%, n=20) were seen in 52 (34.4%) men. ADC was more reproducible (~5% variability). Baseline ADC values did not differ between those with and without measurable growth (p=0.06) but change in ADC did (-6.8±12.3% for those with measurable growth vs. 0.23±10.1% for those without, p=0.0005). A 5.6% reduction in ADC with time indicated a measurable increase in tumor volume (specificity 77.0%, sensitivity 54.9%, AUC=0.67).