The endocannabinoid system modulates neuronal excitability, protects neurons against hyperexcitability and is involved in epileptogenesis in animal models of mesial temporal lobe epilepsy with hippocampal sclerosis. We performed in vivo positron emission tomography imaging of the type 1 cannabinoid receptor in patients with mesial temporal lobe epilepsy with hippocampal sclerosis. Twelve patients with refractory mesial temporal lobe epilepsy due to hippocampal sclerosis received a [(18)F]MK-9470 scan to assess type 1 cannabinoid receptor availability in vivo. Parametric modified standard uptake values were used as quantitative measure of type 1 cannabinoid receptor availability and images were spatially normalized to standard space. Voxel-based analysis was performed comparing patients with hippocampal sclerosis to controls and correlations between type 1 cannabinoid receptor status and seizure characteristics were done using volumes of interest. Type 1 cannabinoid receptor positron emission tomography was co-registered with subtraction ictal single photon emission computed tomography co-registered to magnetic resonance imaging of a complex partial seizure (n = 9). An increased type 1 cannabinoid receptor availability in the ipsilateral temporal lobe was observed, which correlated negatively with the latency since last seizure before scanning and positively to the number of seizures in the month before scanning. A decreased type 1 cannabinoid receptor availability was present in the superior insular cortex, ipsilateral more than contralateral. The ipsilateral insular region displayed a mild ictal hyperperfusion in the transition zone of subtraction ictal single photon emission computed tomography co-registered to magnetic resonance imaging temporal lobe hyperperfusion-frontal lobe hypoperfusion during complex partial seizures. Type 1 cannabinoid receptor availability showed opposite changes in different brain regions that are involved during complex partial seizures in refractory mesial temporal lobe epilepsy with hippocampal sclerosis. The increase in type 1 cannabinoid receptor availability at the seizure onset zone might be a protective mechanism of neurons against hyperexcitability and seizure activity, or contribute to the process of epileptogenesis, or both. The decreased type 1 cannabinoid receptor availability in the insula may play a role in surround inhibition and prevention of seizure propagation.