HLA-A,B,C,DR and DQ antigens were tested in 53 British Caucasian patients with polyarticular onset seronegative juvenile chronic arthritis (JCA); C4 allotypes were also tested in 46. A strong association with HLA-DRw8 was found (RR = 6.1, Fp = 7.6 x 10(-5)), with increased -B5(51) and C4A QO, and decreased -DR7 frequencies. DRw8 incidence correlated with an onset under 5 years, 9 of 12 DRw8+ cases being in this subgroup (Fp = less than 0.06), whereas B5 and C4A QO were prevalent in late onset (greater than or equal to 5 years). Erosions after 5 years associated with HLA-DRw6, and their absence with -Cw1 and -DR5. Genetic susceptibility factors and a further subdivision by onset age are thus demonstrated in this disease. Comparative data suggest that the genetic basis of susceptibility to early onset disease is similar to that of pauciarticular JCA.