A nuclear magnetic resonance (NMR) imaging system is described, and preliminary results from its clinical use are presented. The properties and detection of the magnetisation due to hydrogen protons are outlined, and a rotating frame is introduced to describe the motion of the magnetisation. Radiofrequency (RF) pulses are used to rotate the magnetisation, and slice selection is achieved using a 90 degree RF pulse and a magnetic field gradient. Data acquisition and image reconstruction are explained. Three scanning sequences are described: repeated free induction decay (FID), inversion - recovery, and spin-echo. These sequences produce images whose pixel values have different dependencies on hydrogen proton density, T1 and T2. Inversion-recovery images show striking differentiation between grey and white matter in the brain. The absence of bone artifact is a significant advantage over X-ray computed tomography in the posterior fossa, where rapid repeated FID sequences can also be used to demonstrate flow effects. The considerable soft tissue contrast available with NMR is of value in demonstrating disease within the liver where T1 appears to be sensitive but relatively nonspecific diagnostic parameter. High resolution scans are of value in demonstrating the adrenal gland and spinal cord.