To explore the potential role of the uncoupling protein (UCP) family in human obesity and diabetes, we have used the reverse transcription-polymerase chain reaction to quantify UCP mRNA expression in human skeletal muscle. Levels of mRNA for UCP2, and for both short (UCP3S) and long (UCP3L) forms of UCP3, were highly correlated in individuals, indicating that gene transcription of these UCPs may be coordinately regulated by common mechanisms. In normal glucose-tolerant individuals, muscle UCP2 mRNA levels were positively correlated with percentage of body fat and with BMI (r = 0.6 and P < 0.05 for both). UCP3S mRNA levels were also positively correlated with percentage of body fat (r = 0.52, P < 0.05), and UCP3L mRNA tended to increase as a function of obesity (0.05 < P < 0.1). UCP mRNA levels, however, were not correlated with resting metabolic rate. UCP3S and UCP3L mRNA levels (P < 0.05) and the UCP2 mRNA level (P = 0.09) were increased by 1.8- to 2.7- fold in type 2 diabetes, an effect that could not be explained by obesity. No significant difference was found for UCP2, UCP3S, or UCP3L mRNA levels between insulin-sensitive and insulin-resistant nondiabetic subgroups. We conclude that 1) skeletal muscle mRNA levels encoding UCP2 and UCP3 are correlated among individuals and may be coordinately regulated; 2) UCP3 expression is not regulated by differential effects on UCP3L and UCP3S forms of the mRNA; and 3) UCP mRNA expression tends to increase in muscle as a function of obesity but not of resting metabolic rate or insulin resistance, and is increased in patients with type 2 diabetes.