We report here a new mouse model of multigenic obesity. Backcross progeny ((C57BL/6J x Mus spretus)F1 x C57BL/6J), designated as BSB mice, range from 1% to 50% body fat. Since both parental strains are relatively lean, the wide range of the phenotype in the BSB mice indicates the involvement of multiple genes to produce obesity. Obesity in BSB mice results from increases in both intra-abdominal and subcutaneous fat and is associated with hyperinsulinemia, hyperglycemia, and hyperlipidemia. Female and male BSB mice do not differ in the degree of obesity obtained. Stimulated plasma corticosterone levels are reduced in obese male and female mice. The development of appropriate genetic markers and statistical methods have made it feasible to analyze quantitative polygenic traits in animal models by employing F2 or backcross progeny. Thus, this BSB model is uniquely suited to the genetic analysis of the multifactorial quantitative trait of obesity and its associated phenotypes.