Beta-3-adrenergic receptor (beta-3-AR) and insulin receptor substrate 1 (IRS-1) have been implicated in the pathogenesis of obesity and in obesity related increase in insulin resistance which is associated with, among other diseases, dyslipidemia and type 2 diabetes mellitus. We studied 210 white female Caucasian obese subjects, who underwent a formal weight loss program (Optifast). We examined the association between mutations of the IRS-1 gene at codon 972, mutations of the beta-3-AR gene at codon 64, and the combination of both mutations with the degree of weight loss, waist to hip ratio and the prevalence of hypertension, dyslipidemia and type 2 diabetes mellitus. Twenty-four women (11.4%) were polymorph only for the beta-3-AR mutation, 23 women (10.9%) only for the IRS-1 mutation, and 6 subjects (2.9%) were polymorph for both alleles. No patient displayed a homozygous polymorphism. Similar frequencies of these polymorphisms were observed when the 100 non-obese control women were tested (14.0, 15.0, 3.0, respectively). After 13 weeks of weight loss the group with multiple polymorph alleles had lost less of their weight than the obese controls without mutation (Delta BMI 5.32+/-0.18 versus 6.12+/-0.2 kg/m2, p<0.05). In this group, the frequency of type 2 diabetes (66.7%) was significantly higher than in the obese control group without mutations (16.7%, p=0.008). Our findings suggest there is a synergy between the polymorphisms of Trp64Arg beta-3-AR and Gly972Arg IRS-1 in Caucasian German obese women leading to a decreased weight loss. This seems to be accompanied with an increased frequency of type 2 diabetes.