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1 The N-acetylation of dapsone (DDS) was studied in 108 unrelated Chinese subjects residing in the mainland of China. 2 The frequency of slow acetylators determined using the plasma monoacetyldapsone to DDS ratio (MADDS/DDS, slow acetylators less than 0.30 and rapid acetylators greater than 0.35) at 3 h after an oral dose of DDS (100 mg) was 13.0% (14 of the 108 subjects) with a 95% confidence interval of 7.9 to 20.6%. 3 The mean plasma concentration of MADDS was about three times lower in the slow than in the rapid acetylators and there was a highly significant correlation (rs = 0.886, P less than 0.001) between plasma MADDS concentration and acetylation ratio. 4 Urinary acetylation ratios (MADDS/DDS) and cumulative urinary excretion of MADDS were significantly (P less than 0.001) lower in slow acetylators compared with rapid acetylators. In addition, there was a significant relationship (rs = 0.510 to 0.718, P less than 0.001) between plasma and urinary acetylation ratios. However, the distribution of the urinary acetylation ratio was not bimodal. 5 The urinary acetylation ratio after an oral dose of DDS is not a discriminative index for determining acetylation phenotype.