During a 3-month period, 52 paranoid psychotic inpatients were treated with perphenazine decanoate (Trilafon decanoate) in order to determine a treatment schedule ensuring perphenazine serum levels within the optimal range (2–6 nmol/L). All patients received 1 ml (108 mg) perphenazine decanoate on days 0, 7 and 21, after which the doses and intervals were individually adjusted according to the clinical condition. It was shown that the sum of 2 measurements of perphenazine concentration early during treatment, viz., on days 14 and 21, correlates strongly (r= 0.96) with the steady-state level obtained after 3 months. Consequently, a nomogram for dose and length of interval could be elaborated giving steady-state serum levels of perphenazine of about 4 nmol/L, i.e., in the middle of the optimal concentration range.