Single photon emission computed tomography (SPECT) images were obtained in 3 healthy volunteers and 7 parkinsonian patients 22 h after injection of 120 MBq iodine-123-beta-CIT. There was a high radioactivity in the striatal region against a uniform background in the volunteers but pronouncedly reduced striatal activity in the patients. Total striatal activity in each hemisphere of each individual was calculated as the sum of all activity in excess of the background in all reconstructed images. There was a high correlation between reduction of striatal activity and motor scores in the unified rating scale for parkinsonism (URSP) for the patients. A method developed for calculation of the activity distribution along the length axis of the striatum indicated a more pronounced degeneration in the putamen compared with the caudate nucleus in the patients. Iodine-123-beta-CIT SPECT appears to be a good quantitative method for nigro-striatal degeneration in parkinsonian patients.