Peroxisome proliferator-activated receptor-alpha (PPARalpha) is a nuclear receptor capable of regulating the expression of genes involved in peroxisomal and mitochondrial beta-oxidation pathways. The common Leu162Val polymorphism in the gene encoding PPARalpha has inconsistently shown association with quantitative traits related to obesity, type 2 diabetes, and dyslipidaemia. We genotyped the Leu162Val polymorphism in 1383 patients with type 2 diabetes and 4401 control subjects with normal glucose tolerance (NGT) without showing any association between diabetes and genotype. In addition, the Leu162Val polymorphism was not associated with WHO-defined obesity or dyslipidaemia in case-control settings involving 961 obese and 2563 lean subjects and 1399 dyslipidaemic and 4399 normolipidaemic subjects, respectively. Quantitative trait studies of metabolic variables were carried out in 5799 middle-aged, treatment-naïve subjects showing a difference in fasting serum triglyceride concentrations among homozygous Val-carriers (Leu/Leu+Leu/Val, n=5782, 1.33+/-1.35 mmol/l vs. Val/Val, n=17, 2.22+/-2.4 mmol/l, p=0.007). Similarly, Val/Val was associated with increased fasting serum total cholesterol concentrations (p=0.01). In conclusion, in a relative large-scale study of middle-aged whites we found no evidence of association between the PPARalpha Leu162Val polymorphism and obesity or type 2 diabetes. If replicated, the Val162Val variant may, however, confer an increase in fasting levels of serum lipids.