The authors describe the case of a female patient with the diagnosis of schizophrenia and the CYP2D6 poor metaboliser phenotype (PM phenotype), who experienced severe extrapyramidal side effects, including acute dystonia, while being treated with chloropromazine at 100mg per day (in the third day of therapy). The CYP2D6 phenotype was determined using the sparteine test before and after 3 days of treatment. Metabolic ratio increased 12 times during treatment, from initial 30 to 355. The authors conclude that CYP2D6 poor phenotype leading to slow chloropromazine metabolism, which was further inhibited by chloropromazine during treatment (as chlorpromazine is a strong CYP2D6 enzyme inhibitor) had significant importance on the occurrence of acute extrapyramidal side effects. Most likely the antidopaminergic influence of the drug on the CNS was much more marked due to an inhibition of chlorpromazine metabolism leading probably to an increase of the chlorpromazine blood level.