A group of 26 acute psychotic patients received continuous oral treatment with perphenazine for a period of 5 weeks. Once-weekly blood samples were drawn for measurements of perphenazine levels and, simultaneously, the therapeutic outcome was registered. Another 26 acute psychotic patients received continuous oral treatment with perphenazine for a period of up to 4 weeks. A single blood sample was drawn and the perphenazine concentration was related to the appearance of extrapyramidal side effects. The following conclusions were made: (1) a high risk of provoking extrapyramidal side effects was associated with plasma levels of perphenazine above 3 nmol/l; (2) plasma levels below 2 nmol/l were associated with a poor therapeutic outcome; (3) a 'therapeutic window' between 2 and 3 nmol/l gives maximal therapeutic effect with a low risk of provoking extrapyramidal side effects.