The acetylation of dapsone (DDS) was determined by estimation of the ratio of monoacetyldapsone (MADDS) to DDS concentrations in plasma following a single dose of DDS in 337 white British subjects (193 female; 144 male). The percentage of slow DDS acetylators in the whole group was 60.3%. There was no statistically significant difference in this proportion between 191 elderly subjects (age greater than 65 years) and 143 young subjects (age less than 30 years). Although there was a small (66.3%) but significant (P = 0.033) preponderance of slow acetylators in the young male group there was no difference in the distribution of acetylator phenotypes between the sexes among either the elderly group or in the whole population studied. No correlation was found between absolute body weight and MADDS/DDS ratios.