Our work investigates the relationship between genetic factors and schizophrenia, seeking to identify a gene or genes associated with the clinical form of the disease in a group of Italian patients. In pursuit of the 'dopaminergic hypothesis' of schizophrenia, we explored a possible etiologic role of two dopamine receptor genes, DRD3 and DRD4, that have been repeatedly suggested as factors in the pathophysiology of the disease. We typed DNA polymorphisms in each of the genes that code for variation in the amino acid sequence of the receptor protein. An innovative design using parental chromosomes as controls--the 'haplotype relative risk' strategy--represents a significant improvement over previous association studies in psychiatric genetics. Our results suggest that, at least in our well-defined population, the candidate genes DRD3 and DRD4 do not appear to play a major role in the genetic etiology of schizophrenia.