HAP-1 is a huntingtin-associated protein that is enriched in the brain. To gain insight into the normal physiological role of HAP-1, mice were generated with homozygous disruption at the Hap1 locus. Loss of HAP-1 expression did not alter the gross brain expression levels of its interacting partners, huntingtin and p150glued. Newborn Hap1^−/− animals are observed at the expected Mendelian frequency suggesting a non-essential role of HAP-1 during embryogenesis. Postnatally, Hap1^−/− pups show decreased feeding behavior that ultimately leads to malnutrition, dehydration and premature death. Seventy percent of Hap1^−/− pups fail to survive past the second postnatal day (P2) and 100% of Hap1^−/− pups fail to survive past P9. From P2 until death, Hap1^−/− pups show markedly decreased amounts of ingested milk. Hap1^−/− pups that survive to P8 show signs of starvation including greatly decreased serum leptin levels, decreased brain weight and atrophy of the brain cortical mantel. HAP-1 is particularly enriched in the hypothalamus, which is well documented to regulate feeding behavior. Our results demonstrate that HAP-1 plays an essential role in regulating postnatal feeding.