By means of the image analyzing computer technique, a complete determination of the parenchymal tissue distribution in serially sectioned parathyroid glands were accomplished. The technique had good reproducibility. Taking into account the shrinkage of the different tissue components during histotechnical procedures and the tissue densities, it was possible to calculate the parenchymal cell mass of unfixed glands. The cell distribution varied considerably, and in most glands as many as 10 sections at different levels had to be examined to get a reliable ratio between the parenchymal and fat cell tissue. The results seriously question the validity of histopathologic examination of one or a few sections of parathyroid glands in evaluation of the parenchymal cell mass, as well as diagnoses based on examination of partial glandular biopsy specimens.