Schizophrenia is one of the most severe mental disorders. Heredity is accepted as a major causative factor. To find molecular mechanisms behind schizophrenia, patient materials with reliable and valid diagnoses must be identified. In order to compare schizophrenia diagnostic procedures for reliability, validity and suitability for genetic studies by evaluation of record information, interview and register diagnostic data and to examine patient materials for linkage or association with molecular genetic markers three patient materials were recruited: sporadic cases, a large pedigree and sib-pairs. Schizophrenia diagnoses based on patient records only, showed good to excellent agreement with diagnoses based on both records and interviews. Register diagnoses generally displayed poor agreement with research diagnoses, but in 94% of patients sometimes registered as schizophrenic psychoses a research diagnosis of these disorders was certified. In the pedigree, analysis suggested linkage to chr 6p23 in a single branch of the pedigree, and a genome scan indicated linkage to the 6q25 region. A genome scan analysis of the sib-pair material was suggestive of linkage to chr 10q25.3-q26.3. In the case-control sample and a meta-analysis there was an association between a dopamine D2 receptor polymorphism (Ser311Cys), on chr 11q22-23, and the disorder. Brain-derived neurotrophic factor gene variants (chr 11p13) were also analysed without any robust significant findings. For patients in long-term treatment for schizophrenia in Sweden, psychiatric record reviews should be valid, reliable and sufficient for assessment of lifetime research diagnosis. Swedish register diagnosis of schizophrenic psychoses has a high positive predictive power in relation to corresponding research diagnoses. For future Swedish studies focusing on a broad definition of schizophrenia, it is sufficient to rely on the register diagnoses of schizophrenic psychosis. There is no major vulnerability gene or locus that is common to the majority of patients with schizophrenia, indicating genetic heterogeneity.