Glutamatergic receptor expression is mostly unknown in adults with fragile X syndrome (FXS). Favorable behavioral effects of negative allosteric modulators (NAMs) of the metabotropic glutamate receptor subtype 5 (mGluR₅) in fmr1 knockout (KO) mouse models have not been confirmed in humans with FXS. Measurement of cerebral mGluR₅ expression in humans with FXS exposed to NAMs might help in that effort. We used positron emission tomography (PET) to measure the mGluR₅ density as a proxy of mGluR₅ expression in cortical and subcortical brain regions to confirm target engagement of NAMs for mGluR₅s. The density and the distribution of mGluR₅ were measured in two independent samples of men with FXS (N = 9) and typical development (TD) (N = 8). We showed the feasibility of this complex study including MRI and PET, meaning that this challenging protocol can be accomplished in men with FXS with an adequate preparation. Analysis of variance of estimated mGluR₅ expression showed that mGluR₅ expression was significantly reduced in cortical and subcortical regions of men with FXS in contrast to age-matched men with TD.