This study was designed to investigate the relationship between the treatment effect of olanzapine and the serum prolactin level in schizophrenia and to investigate the factors that may act as predictors of response for olanzapine treatment.Sixty patients who met the DSM-IV criteria for schizophrenia were included in the study. None of the patients were drug-naive, and they were given olanzapine in a flexible dose of 10-30 mg/day for 3 months after a 7-day drug washout period. Serum prolactin levels were measured at baseline (after drug washout) and at months 1, 2, and 3 during olanzapine treatment. A psychiatrist performed monthly ratings of symptoms using the Positive and Negative Syndrome Scale Manual (PANSS Manual). The Generalized Estimating Equations-I was used for data correlation analysis. Data were gathered from July 2005 to July 2006.In general, the serum prolactin level was decreased in schizophrenia patients with olanzapine treatment, although the difference is not statistically significant (p = .974, p = .246, and p = .363 for the first, second, and third months, respectively). There was a close relationship between the improvement in positive symptoms and the change in serum prolactin levels before and after olanzapine treatment (p = .002). Moreover, the serum prolactin level also had a positive association with female gender (p = .008). The present study demonstrated no significant correlation between serum prolactin level, MAOA polymorphism, and DRD4 genotype.This finding suggests that the serum prolactin level may be a useful biological marker to predict the effectiveness of antipsychotics in schizophrenia.