Serum haloperidol levels were determined in 59 patients, 50-88-years-old, with psychosis, receiving long-term treatment with haloperidol. Although the total (bound and free form) haloperidol level in serum showed a linear correlation with daily dose, there was a larger variation in the relationship between free form and the daily dose compared with total because of inter-individual variation in the serum protein binding of haloperidol. The free fraction of haloperidol in serum increased with age. There was no difference in the ratio of total haloperidol level per daily dose between the adult and elderly groups, whereas the ratio of free haloperidol level per daily dose was significantly higher in the elderly than in the adult group. In the elderly, therefore, the therapeutic window of haloperidol should be assessed using free form level rather than total level, which is influenced by serum protein binding of the drug.