Urinary lipoperoxides, measured as the malondialdehyde-thiobarbituric acid adduct, were quantified by adapting to urine the liquid-chromatographic method of Wong et al. (Clin Chem 1987;33:214-20) to plasma. Reference intervals for untimed urine specimens from 121 men, ages 16 to 67 years, and 107 women, ages 15 to 55, were determined. Their concentrations differed significantly (P = 0.015), males having a mean (and SD) of 0.89 (0.35) nmol of malondialdehyde per milligram of creatinine, females 0.78 (0.30). In both groups, the values were slightly skewed to the higher values. Our early studies suggest that measuring urinary lipoperoxide may have advantages over plasma in studying certain disorders. The presence of other urinary chromophores or TBA-reactive substances stresses the need for chromatographic techniques when lipoperoxides are measured in biological samples.