Most monogenic cases of obesity in humans have been linked to mutations in genes encoding members of the leptin–melanocortin pathway. Specifically, mutations in MC4R, the melanocortin-4 receptor gene, account for 3–5% of all severe obesity cases in humans^{, –}. Recently, ADCY3 (adenylyl cyclase 3) gene mutations have been implicated in obesity^,. ADCY3 localizes to the primary cilia of neurons, organelles that function as hubs for select signaling pathways. Mutations that disrupt the functions of primary cilia cause ciliopathies, rare recessive pleiotropic diseases in which obesity is a cardinal manifestation. We demonstrate that MC4R colocalizes with ADCY3 at the primary cilia of a subset of hypothalamic neurons, that obesity-associated MC4R mutations impair ciliary localization and that inhibition of adenylyl cyclase signaling at the primary cilia of these neurons increases body weight. These data suggest that impaired signaling from the primary cilia of MC4R neurons is a common pathway underlying genetic causes of obesity in humans.