Fifteen depressed elderly patients (14 female, 1 male; mean age 85 years) received a single oral dose of amitriptyline. The concentration of amitriptyline plus nortriptyline in a blood sample taken 24 hours later was used to predict by means of a nomogram the amitriptyline dosage required for each patient. Each dose was selected to produce steady-state amitriptyline plus nortriptyline concentrations close to 140 micrograms/L. The daily dosage ranged from 20 to 100mg (mean 62mg). Patients received the individually calculated dose each night, and weekly blood samples were obtained for drug analysis. At 2 weeks, mean amitriptyline plus nortriptyline concentrations were 118 +/- 21 micrograms/L. Eight of the patients were studied for a further 2 weeks and the mean amitriptyline plus nortriptyline concentration was then 111 +/- 19 micrograms/L. The dose prediction test is easy to use and ensures each patient receives an adequate but safer dose of amitriptyline than might otherwise be prescribed routinely.