Serotonin transporter gene is known to be implicated with various psychological and psychiatric phenotypes. The present study aimed at searching for relationship of the gene polymorphism 5-HTTLPR to personality traits and P300 wave in patients with schizophrenia (n = 71) and their non-psychotic relatives (n = 74). To reduce an influence of the P300 amplitude range, which may vary widely in population, subjects have been divided into 2 groups: with amplitude higher (group m+) and lower (group m-) than a mean sample value. Between-amplitude differences were highly significant (p < 0.0001) for all the leads. The groups were similar by age, sex ratio and emotional level measured with personality inventories. When compared by 5-HTTLPR genotype, mean P300 amplitude values were similar in the group m-. In the group m+, significant differences for this component were found between the II genotype and the ss genotype at frontal (p = 0.02) and central (p = 0.005) leads, with the ss genotype subjects having the lowest amplitude. Subjects with the ss genotype in this group as compared to those with the II genotype were also featured by higher anxiety- and depression-related personality traits in relatives and marked clinical symptoms related to anxiety and depression in patients. The fact that association between 5-HTTLPR polymorphism and with P300 amplitude as well as personality traits was found only in the group with higher amplitudes, but not in the group with lower amplitudes, may be partly explained by the assumption that these groups represent different phenotypes. Subjects with inherent lower amplitude are thought to have reduced ability of behavior self-regulation. In conclusion, 5-HTTLPR polymorphism seems to contribute to information processing related to P300 generation and to anxiety and depression traits in patients with schizophrenia and their relatives.