Mean diffusivity (MD) in tumors is often too readily interpreted as cellularity. Here, we investigated which microstructural features explain the MD in meningioma tumors. We performed high-resolution MRI and histological imaging on excised mengioma tumor samples and coregistered MD to histology. We found that cellularity alone was a poor predictor of MD, and that the prediction improved markedly by considering also the microcyst density. A qualitative analysis also indicated that collagen fibers, stroma and psammoma bodies affect the MD.