The type 1 cannabinoid receptor (CB1) is a crucial modulator of synaptic transmission in the brain. Animal and postmortem human data suggest that mutant huntingtin represses CB1 transcription. Our aim was to measure CB1 levels in the brains of Huntington disease (HD) patients in vivo. Twenty symptomatic HD patients and 14 healthy controls underwent PET with the novel CB1 ligand N-[2-(3-cyano-phenyl)-3-(4-(2-¹⁸F-fluorethoxy)phenyl)-1-methylpropyl]-2-(5-methyl-2-pyridyloxy)-2-methylproponamide. We observed a profound decrease of CB1 availability throughout the gray matter of the cerebrum, cerebellum, and brain stem in HD patients, even in early disease stages. Disease burden ([number of CAG repeats in the HTT gene – 35.5] × age) was inversely correlated with CB1 availability in the prefrontal and premotor cortex. The profound early and widespread reduction of CB1 availability in vivo is consistent with the hypothesis that mutant huntingtin represses CB1 transcription. This is the first, to our knowledge, in vivo demonstration of disturbance of the endocannabinoid system in a human neurologic disease.