Abstract We investigated the effect of gender on plasma prolactin levels in 20 Japanese drug-naïve schizophrenic patients [10 male, 10 female, aged 25.4 ± 10.3 (mean ± S.D.), range = 12–46 years] treated with olanzapine. Plasma prolactin levels were measured at baseline, and weeks 3 and 8 after starting titration of olanzapine. Comparisons of plasma prolactin levels between baseline and week 3, and between baseline and week 8 were made by repeated analysis of variance (ANOVA) and paired t-test. Two-way ANOVA showed a significant difference in olanzapine-induced prolactin changes between male and female patients ( P = 0.037). In male patients ( n = 10), the plasma concentration of prolactin at week 3 was significantly higher than at baseline ( P = 0.016), but there was no significant difference between the plasma concentration of prolactin at week 8 and at baseline or week 3 ( P = 0.191). In female patients ( n = 10), there was a significant change of prolactin between baseline and week 3 ( P = 0.005), and between baseline and week 8 ( P = 0.047). Our results indicate the possibility of gender differences in prolactin elevation induced by olanzapine in Japanese drug-naïve schizophrenic patients. These gender-based findings may be helpful for clinicians when deciding the frequency of follow-up visits once a patient starts olanzapine therapy.