OBJECTIVE The heterogeneity of schizophrenia mainly results from variations in clinical expressions of the disease, such as age at onset, gender differences in onset of illness, symptoms and response to antipsychotic treatment. Enhanced sensitisation of dopamine pathways in males, having consistently an earlier onset, might be implicated as disease modifiers for schizophrenia in males. METHODS In this study, we performed a case (n = 87)-control (n = 100) association study between the DBH5'-ins/del and DBH-444g/a polymorphisms of the DBH gene and also compared the level of psychotic symptoms between patients with different DBH genotypes/haplotypes with respect to antipsychotic therapeutic response and gender difference. RESULTS No significant differences between allele and genotype and haplotype frequencies at either groups (p < 0.05). When the age is considered in patient group, a significant difference was observed between patients with ID genotype and with II genotype (p = 0.018). Patients with ID genotype have been diagnosed as schizophrenics in early ages when compared to II genotype carriers. We also found a significant difference between II and ID genotype (p = 0.007) when the gender had taken into account, showing that the ID genotype carriers had an early onset to schizophrenia. CONCLUSIONS This association was more significant in male schizophrenia patients than females. Thus, this finding may constitute a novel biological support for the prior finding that onset of schizophrenia varies with gender. The results also showed that critical genetic vulnerability may be associated with the presence or absence of the ID genotype of DBH5'-ins/del.