This study aims to assess type of melanin as a risk indicator for skin tumors, in a sample of melanoma cases and controls within a larger multicenter study (Helios 2), held in Europe and South America in 2001-2002. In each case and control, the melanin content in hair was assessed by three methods: 1) the amount of 2,3,5-pyrroletricarboxylic acid (PTCA); 2) the absorbance ratio with ultraviolet spectroscopy; and 3) the spectra of near-infrared spectroscopy. Statistical analysis was performed in a Bayesian setting, defining priors for confounders and effect modifiers from the larger study data set. Subjects with values of PTCA of less than 85 ng/mg carried an increased risk (26 vs. seven discordant pairs: odds ratio = 4.4, 95% confidence interval: 1.52, 14.54), adjusted by hair color, eye color, and number of nevi (n = > or =40). The absorbance ratio showed a weaker and nonsignificant odds ratio of 1.5. After correction by misclassification, near-infrared spectroscopy was associated with an odds ratio of 2.3 (95% confidence interval: 1.36, 4.22). The amount of PTCA is thus a strong and independent risk indicator for melanoma. Incorporating PTCA determination into epidemiologic studies is therefore recommended.