The psychopathology of Alzheimer's disease (AD) is varied and includes both behavioural and psychological symptoms. Behavioural and psychological symptoms are common and contribute to the difficulties experienced by carers. However, the mechanism whereby these symptoms occur in some individuals with AD is not understood. We hypothesized that common genetic polymorphisms in neurotransmitter systems are risk factors for these symptoms in the course of AD. A total of 211 subjects from a population-based prospective study of psychopathology within late-onset AD were genotyped for the 5-HT2A receptor polymorphism 102-T/C and the 5-HT2C receptor polymorphism Cys23Ser. Associations were found between the presence of the C102 allele and the presence of visual (Fisher's exact test, one-tailed, P = 0.003) and auditory hallucinations (Fisher's exact test, one-tailed, P = 0.004) and between the presence of the Ser23 allele and visual hallucinations (chi2 = 7.5, df = 1, P = 0.006) (P = 0.03, 0.04 and 0.06, respectively, after Bonferroni correction). In addition, there was an association between the Cys23Ser polymorphism and hyperphagia (chi2 = 6.7, df = 2, P = 0.03) (P = 0.3 after Bonferroni correction). We conclude that common 5-HT2A and 5-HT2C genetic polymorphisms previously showing only weak associations with psychotic illness are associated with psychotic symptoms in AD but are clinically silent until the onset of the neurodegenerative process.