This trial was conducted to study the frequency of the slow acetylator phenotype in asymptomatic HIV patients having no previous reaction to sulfa-drugs, and to compare this frequency with the frequency found in healthy controls. Results show that HIV alone is not capable of modifying the acetylator phenotype; the prevalence of slow acetylator phenotype is the same in immune competent subjects and HIV-positive patients. It is more common in HIV-positive patients with a CD4+ lymphocyte count of less than 200 mm-3.