Abstract: Heterozygous coding mutations in the melanocortin 4 receptor (MC4R) are implicated in 1 to 6% of early onset or severe adult obesity cases. To better address the problem of the genotype:phenotype relationship within this specific form of obesity, we systematically studied the functional characteristics of 50 different obesity‐associated MC4R mutations. Structure modeling of MC4R indicates that obesity‐associated MC4R mutations are not localized in a single domain of the protein. We developed a flow cytometry‐based assay to compare cell membrane expression of obesity‐associated MC4R mutants. Using this assay, we demonstrate that over 54% of the obesity‐associated MC4R mutations impair the membrane expression of MC4R. All other mutations impair the basal constitutive activity and/or the EC₅₀ for the physiological agonist α‐MSH as measured in a cAMP‐ dependent luciferase assay. The extent of the alterations in receptor activity ranges from a total suppression of MC4R activation in response to α‐MSH to a mild alteration of the basal constitutive activity of the receptor. Since most patients are heterozygous for MC4R mutations, these data indicate that a small decrease in overall MC4R activity can cause obesity, strongly supporting the hypothesis that the MC4R is a critical component of the adipostat in humans.