The FDA-approved starting dosage of capecitabine is 1,250 mg/m(2), and market research indicates that U.S. physicians routinely prescribe 1,000 mg/m(2). Retrospective analyses however report reduced toxicity and efficacy in a subset of patients with the 3R/3R genotype of the thymidylate synthase gene enhancer region (TSER). This study sought to develop TSER genotype-specific guidelines for capecitabine dosing. Capecitabine was dose-escalated in advanced and/or metastatic cancer patients with TSER 3R/3R (Group A; N = 18) or 2R/2R + 2R/3R (Group B; N = 5) from 1,250 to 1,625 mg/m(2) b.i.d., every 2 weeks on/1 week off for up to 8 cycles. Parent and metabolites pharmacokinetics, adverse events, and tumour response were assessed. The maximum tolerated and recommended doses in 3R/3R patients are 1,625 mg/m(2) and 1,500 mg/m(2). At 1,500 mg/m(2), one in nine 3R/3R patients experienced a dose-limiting toxicity. Dosing guidelines for 2R/2R + 2R/3R remain undetermined due to poor accrual. The results indicate that 3R/3R patients may be amenable to 1,500 mg/m(2) b.i.d. on an intermittent schedule, and is the first to prospectively validate the utility of TSER pharmacogenetic-testing before capecitabine treatment.