Forty-three patients with seropositive, erosive rheumatoid arthritis (RA) and 24 members of 5 RA multicase families were studied for HLA class I1 gene polymorphism, using restriction fragment analysis with complementary DNA probes for DRP and DQP chains. This method generates HLA-DR-DQ haplotypes that are highly correlated with HLA-DR serology. Thirtyfive of the 43 RA patients (81%) were positive for one or for both of the DR4-associated DR-DQ haplotypes, 4.1 and 4.2. Among these patients, the 4.1 haplotype was found significantly more often than in DR4 + controls (P < 0.01). The haplotype segment C3;B15;DR4 was present in all RA patients in 4 of the 5 families, and included the DR-DQ4.1 haplotype.The etiology of rheumatoid arthritis (RA) is still unknown. However, for a long time, a constitutional basis for susceptibility to the disease has been implicated, and this is strongly supported by the finding of Address reprint requests to Johan Wallin, Center for Biotechnology, F82, Huddinge Hospital, S-141 86 Huddinge, Sweden.Submitted for publication November 26, 1986; accepted in revised form May 27, 1987. an association between RA and HLA-Dw4 (1). This specificity is defined using homozygous typing cells. When serologic means of studying class I1 mixed lymphocyte culture (MLC) antigens were introduced, findings clearly indicated that RA was closely associated with HLA-DR4. More recently, additional class 11 loci have been defined using both serologic and cellular typing techniques. Three of these loci contained genes encoding polymorphic products and are now referred to as DP, DQ, and DR.DR antigens have been well characterized using serologic methods, and so far, 12 distinct allelic products have been defined. The DQ locus is closely linked to DR; the DQ alleles exist in strong linkage disequilibrium with particular DR alleles. Serologic typing has only revealed 3 distinct DQ allotypes: wl, w2, and w3. Linkage disequilibrium between DR and DQ is exemplified by the fact that almost all DR4 + individuals are also DQw3 + .New methods have recently been used to study the association between HLA and diseases. Complementary DNA (cDNA) probes have been used to genotype patients for class I1 alleles. A method based on restriction fragment length polymorphism (RFLP) has been developed (2) that, together with recent findings (3), gives a solid basis for the assignment of genetically defined DR-DQ haplotypes, which correlate extremely well with the serologic DR specificities. DRP-specific RFLP gives a resolution similar to that which can be observed when serologic reagents are used. However, genotyping shows a higher degree of polymorphism for the DQ locus than was determined from earlier serologic studies. In 1985, Bohme et a1 (2) defined 7 distinct DQP subtypes. The association between HLA class I1 alleles and insulin-dependent