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To establish whether gender or N-acetyltransferase 2 (NAT2) genotype influence the urinary 17 U + 17X/137X ratio after dosing with caffeine.Ninety-two nonsmoking individuals underwent caffeine phenotyping. NAT2 genotype was determined by the polymerase chain reaction followed by a restriction digest (PCR-RFLP).The median ratio for urinary 17 U + 17X/137X was 6.7 (range 1.45-18.65). 55% of subjects were slow acetylators. Gender did not affect the metabolic ratio or NAT2 genotype. Mean 17 U + 17X/137X ratio differed between fast (6.75) and slow (8.69) acetylators (95% CI for the difference, 0.32-3.56).The findings are further evidence that the 17 U + 17X/137X urinary ratio is not a robust measure of CYP1A2 activity. A possible mechanism by which the ratio might be influenced by NAT2 genotype is suggested.