Recent studies have suggested an association between Type II (non-insulin-dependent) diabetes mellitus-related phenotypes and a cytosine-to-thymidine substitution that results in the replacement of tryptophan by arginine at codon 64 (Trp64Arg or W64R) of the β ₃-adrenergic receptor gene. Here, we present the results of possibly the largest association study to date on the variant in a sample of 526 families with a total of 1725 subjects, 1053 of whom had Type II diabetes. Preliminary calculations suggested that we had excellent power to detect the moderate associations which were reported in previous studies. No associations were found between the W64R variant and the following phenotypes in our sample: Type II diabetes, age at diagnosis for Type II diabetes, measures of obesity, fasting glucose, fasting insulin, minimal model variables, and systolic and diastolic blood pressures. In the analysis of plasma lipids, we detected an association between the variant and HDL ratios (HDL cholesterol/total cholesterol) (p = 0.013), which remained significant even after adjusting for sex, affection status and age. Since W64R homozygotes (n = 11) had the highest HDL ratios, however, heterozygotes had the lowest and the wild-type subjects had intermediate values, we conclude that the W64R variant is unlikely to reduce HDL ratios in a dose-dependent, pathogenic manner. [Diabetologia (1999) 42: 238–244]