We evaluated whether cytochrome P450 (CYP) poor metabolizer polymorphisms of CYP2D6 and CYP2C19 are relevant for the outcome (measured by length of hospitalization) during treatment with psychotropic medications in patients with depression or schizophrenia. 229 patients were genotyped by real-time PCR hybridization probe melting curve technique for CYP2C19*2, CYP2D6*3, *4, and *6, respectively. The gene deletion CYP2D6*5 was analyzed by a long PCR method. Detailed clinical information was obtained from 53 subjects. Patients genotyped homozygous or heterozygous for those CYP2D6 and CYP2C19 poor metabolizer alleles were treated for a longer time in hospital (median 57.5 vs. 40.0 days). Psychiatric patients might benefit from CYP genotyping, the duration of stay as inpatient might be reduced by a priori selection of the appropriate drug for the individual patient.