Purpose: The purpose of this study was to quantify the variability of Apparent Diffusion Coefficient (ADC) and test if there were statistically significant differences in ADC between MRI systems and sequences. Method: With a two-chamber cylindrical ADC phantom with fixed ADC values (1,000 and 1,600x10⁻⁶ mm²/s) a single-shot (ss) Echo Planar Imaging (EPI), a multi-shot EPI, a reduced field of view DWI (zoom) and a Turbo Spin Echo DWI sequence were tested in six MRI systems from three vendors at 1.5 T and 3 T. Technical parameters were according to Prostate Imaging Reporting and Data System Version 2.1. ADC maps were calculated by vendor specific algorithms. Absolute and relative differences in ADC from the phantom-ADC were calculated and differences between sequences were tested. Results: At 3 T absolute differences from phantom given ADC (∼1,000 and ∼ 1,600x10⁻⁶ mm²/s) were −83 – 42x10⁻⁶ mm²/s (-8.3%-4.2%) and −48 – 15x10⁻⁶ mm²/s (-3%-0.9%), respectively and at 1.5 T absolute differences were −81 – 26x10⁻⁶ mm²/s (-2.6%-8.1%) and −74 – 67x10⁻⁶ mm²/s (-4.6%-4.2%), respectively. Significant statistical differences in ADC measurements were identified between vendors in all sequences except for ssEPI and zoom at 3 T in the 1,600x10⁻⁶ mm²/s phantom chamber. Significant differences were also identified between ADC measurements at 1.5 T and 3 T in some of the sequences and vendors, but not all. Conclusion: The variation of ADC between different MRI systems and prostate specific DWI sequences is limited in this phantom study and without apparent clinical relevance. However, prospective multicenter studies of prostate cancer patients are needed for further investigation.