Serum concentrations of cis(Z)-flupentixol have been measured in patients on cis(Z)-flupentixol decanoate injections during successive dosage intervals of 2-4 weeks. The calculation of the fluctuation of the serum concentration in the dosage interval indicates that each individual patient should have his own dosage interval. For the 2-week group a significant correlation was found between weekly dose and preinjection concentration (r = 0.79) although an interindividual variability of X 4 was found. For this group the relative within-patient variation in the pre-injections concentration was calculated to be 9.6%, showing that a constant dosage regimen in the individual patient leads to an almost constant drug load.