The purpose of this study was to evaluate diffusion-weighted MR imaging in neuroblastomas. We prospectively examined seven children (age range 1-3 years) with seven solid body neuroblastomas. Diagnosis was established histologically. Diffusion-weighted echo-planar imaging (EPI) sequence was performed in all patients, with a repetition time of 5400 ms and an echo time of 103 ms, and with a b-value of 1000 s/mm(2). The contrast of tumour tissue depicted with T2-weighted images and diffusion-weighted images were evaluated by means of region-of-interest measurements and a calculation of the apparent diffusion coefficient (ADC) was done. The ADC calculation showed a mean ADC of 1.1x10(-3) (SD 0.14x10(-3), range 0.9-1.2x10(-3)) mm(2)/s of all tumours. Diffusion-weighted images showed an increased tumour signal. Water proton diffusion within the tumour matrix of neuroblastomas is especially restricted by the molecular and macromolecular barriers due to the very dense structure of this tumour tissue. We hypothesize that high nuclear-to-cytoplasm ratio of neuroblastoma cells limits intracellular motion. Furthermore, the very densely packed tumour cells inhibit effective motion of extracellular water protons. Restricted proton motion leads to a reduction in the rate of apparent diffusion and to a marked increase in signal on diffusion-weighted EPI MR images.