Theoretical concepts underlying the principle of nuclear magnetic resonance imaging are defined and the technical basis for its use in medical imaging described (sequences, accumulations, contrast media). Results are presented of a spectrometric study of 23 operative specimens from the bladder, prostate and seminal vesicles with analysis in vitro of relaxation time of the various tissues. Findings confirmed the value of NMR imaging for evaluation of macroscopic extension of tumors within the affected organs, as well as the interrelations of the organs, but the impossibility of identifying histologic structure of tumors and organs explored by this technique. Two groups of subjects were studied: a first group of 20 healthy volunteers to define NMR semeiology of male pelvis and a second group of 40 patients with mainly tumoral lesions of vesicles or prostate. Details are provided of the diagnostic value of NMR imaging with respect to the tumor, local and regional invasion and bone metastases. Excellent images of the tumor were obtained, with the possibility of determining its macroscopic stage, but its histologic nature could not be ascertained. Multiplane examination allowed better assessment of local and regional spread than by a CT scan, particularly for prostatic tumors (bladder, prostate bed or seminal vesicle extension). A negative feature of major importance exists, however, in that early glandular involvement is not detected by NMR imaging, which also fails to identify one case in two with bone metastases. Current research should allow progressive correction of present NMR imaging insufficiencies, notably in the field of tissue characterization.