In the authors' Medullary Screening Clinic, medullary thyroid cancer (MTC) or C-cell hyperplasia (CCH) has been identified in 26 relatives of 12 apparently "sporadic" patients from 56 families in which calcitonin (CT) testing has been performed. The interpretation of stimulated test results was sometimes difficult: elevated basal levels without a significant increase after the injection of secretagogues; basal levels in the normal range with considerable increases after stimulation but not exceeding two or three times the upper limit of normal; elevated basal or stimulated CT levels that slowly decreased to normal or near-normal levels over relatively short periods of time. The latter results suggest the possibility of CCH disappearing spontaneously in some patients.