Influence of genetic factors in the spondylarthropathies has been clearly established. The goal of familial studies is to determine the respective weight of genetic factors and to discover new genes involved in the pathogenesis of spondylarthropathies. Among these factors, the HLA region appears to be of most importance. We are currently conducting a prospective multicenter French study aimed at collecting multiplex families of spondylarthropathies. We present preliminary results obtained from nearly 300 individuals corresponding to the first 20 collected families. Through the screening procedure, we have identified 57 patients of an average age of 42 and a mean age at onset of 23. The relative risk of spondylarthropathy in males as compared with females is 1.03. HLA typing revealed the presence of HLA-B27 in all affected patients from 19 of the 20 families, whereas in the last family, the two affected individuals shared the HLA-B14 allele. Study of HLA class I allelic transmission confirmed a strong association of spondylarthropathy with HLA-B27 (p < 10(-9)). A weaker association was found with HLA-A28, HLA-C1 and HLA-C2, probably secondary to linkage disequilibrium between these alleles and HLA-B27. Study of HLA class II region indicated a possible protective effect of HLA-DR2 and an influence due to the presence of HLA-DR14. The risk to develop spondylarthropathy in HLA-B27+ relatives of patients decreased very rapidly in respect to the genetic distance, suggesting the genetic contribution of at least one other important gene distinct from the HLA region. These data encourage us to carry on a systematic screening of the whole human genome to discover implicated genes.