The prevalence of mutations within and in the flanking regions of the gene encoding the melanocortin 4 receptor was investigated in severely obese and normal-weight subjects from the Swedish Obese Subjects study, the Health, Risk Factors, Exercise Training, and Genetics (HERITAGE) Family study, and a Memphis cohort. A total of 433 white and 95 black subjects (94% females) were screened for mutations by direct sequencing. Three previously described missense variants and nine novel (three missense, six silent) variants were detected. None of them showed significant association with obesity or related phenotypes. In addition, two novel deletions were found in two heterozygous obese women: a -65_-64delTG mutation within the 5' noncoding region and a 171delC frameshift mutation predicted to result in a truncated nonfunctional receptor. No pathogenic mutations were found among obese blacks or nonobese controls. Furthermore, none of the null mutations found in other populations was present in this sample. In conclusion, our results do not support the prevailing notion that sequence variation in the melanocortin 4 receptor gene is a frequent cause of human obesity.