Examination of the pattern of hereditary predisposition allows a more precise determination of pathogenetic relationships between individual clinical forms and variants of multifactorial diseases (MFD), as well as assessment of the possibilities and approaches to a genetic classification to be made. The construction of genetic classifications is based on the identification of differences in the type of inheritance, the study of the progeny in families where both parents have the same or different forms of the disease, the results of the marker adhesion test, and the genetic correlation coefficients. The study has made use of some clinical forms of rheumatic diseases, whose relationships are a subject of controversy among the clinicians, as splitting phenotypes. Segregation analysis of the selected clinical forms of rheumatic diseases and genetic correlation coefficients obtained within the framework of a quasi-continuous model provide no indication that these forms can be isolated as independent nosologic entities. Possible phenotype splitting into individual subtypes and the potentialities of a marker approach to the construction of genetic MFD classifications are demonstrated, with rheumatoid arthritis associated with HLA antigen Dr 4 taken as an example.