The multi-institutional ACRIN 6698 trial investigates quantitative diffusion weighted imaging (DWI) for assessment of breast cancer response to treatment. A secondary aim is to perform a test/retest study to evaluate reproducibility of ADC measures. In a subset of 91 subjects, DWI was acquired twice in the same exam, before and after patient repositioning. Tumor ADC was measured separately for each acquisition. 80% of cases were found analyzable by a standardized quality-control procedure. Reproducibility of ADC was excellent, with intraclass correlation coefficient of 0.97 (95%CI 0.95, 0.98) and agreement index of 0.83 (95%CI 0.76,0.87). Results were similar for subgroups by field strength or study visit.