Using receiver operating characteristics (ROC) analysis, the clinical relevance of urinary delta-aminolevulinic acid concentration adjusted to the logarithm of creatinine concentration (ALA/log.Cn) proved to be better than that of uncorrected ALA or ALA/Cn ratio for the detection of blood lead levels (Pb-B) exceeding 2.5, 3.0, or 3.5 mumol/l in 483 men occupationally exposed to lead. Ala/log.Cn yielded validities of 1.63, 1.75, and 1.79 at Pb-B levels 2.5, 3.0, and 3.5 mumol/l, respectively. The same values for ALA/Cn were 1.59, 1.70, 1.78 and for ALA 1.52, 1.65, 1.72, respectively. The simple adjustment to log. Cn makes urinary ALA determination more efficient in biological monitoring of the effect of occupational lead exposure.