I wish to point out a significant error in the article by Peter W. Mullen, “Optimal phenytoin therapy: A new technique for individualizing dosage,” which appeared on page 228 of the February, 1978, issue of the Journal. In Table I, the serum phenytoin concentrations he predicted by using the method described by Ludden and colleagues (Clin. Pharmacol. Ther. 21:287–293, 1977) for Patient C are given as 9.7 and 24.3 mg/L for the 200 and 300 mg/day doses, respectively. I obtain 7.6 and 31.5 mg/L, respectively, using Ludden and co‐workers' method.