Increased dopamine D2 receptor binding in basal ganglia has been reported in human narcolepsy. These studies have been based on post-mortem material of 8 patients, most of them also medicated for narcolepsy. We studied six narcoleptics without stimulant or anticataplectic medication. The patients had an unambiguous history of cataplexy, and they were also studied polygraphically. Single photon emission computed tomography (SPECT) imaging was performed. The D2 receptor density was determined by using 123I-iodobenzamide (IBZM). The control subjects were 8 unmedicated Parkinson patients with one-sided (hemiparkinsonian) clinical symptoms. The D2 receptor density in them is known to be normal or somewhat increased compared to healthy normals. The striatum/frontal D2 activity ratio was 1.331 +/- 0.084 (with phantom study correction 2.101 +/- 0.300) in the narcoleptic patients, and in the parkinsonian controls 1.321 +/- 0.052 (2.067 +/- 0.185) for the asymptomatic side and 1.335 +/- 0.025 (2.117 +/- 0.090) for the symptomatic side (i.e. contralateral to the side with the clinical extrapyramidal signs). There was no statistical difference between the groups or between the symptomatic and asymptomatic side in the Parkinson patients. Thus, our results differ from the earlier post-mortem studies.