Amonafide is extensively metabolized, including N-acetylation to an active metabolite. Prior studies have demonstrated that patients who are fast acetylators of amonafide (and other drugs) have increased toxicity at standard doses of amonafide. The primary objective of this study was to define the recommended phase II dose of amonafide separately for slow and fast acetylators. Twenty-six patients with advanced cancer underwent acetylator phenotyping with caffeine and were assigned to a dose level. Slow acetylators were treated at 375 mg/m2 (daily for 5 days) and had a median WBC nadir of 1600/microliters. Fast acetylators were treated at both 200 and 250 mg/m2, resulting in median WBC nadirs of 5300 and 2000/microliter, respectively. Two patients were not typeable, and two patients appear to have been misphenotyped, one in each phenotype category. Pharmacodynamic analysis yielded a model for nadir WBC including acetylator phenotype, 24-h N-acetyl-amonafide plasma concentration, gender, and pretreatment WBC. We recommend doses of 250 and 375 mg/m2 (for 5 days) for further phase II testing of amonafide in fast and slow acetylators, respectively.