Twelve families with a sibling pair affected by seronegative juvenile chronic arthritis (JCA) were studied. The ratio of HLA haplotype sharing was significantly different from that expected. No common haplotype was found. All 10 sibling pairs concordant for pauciarticular disease shared 2 haplotypes and the other 2 pairs shared one haplotype. These findings are evidence of a genetic predisposition to develop pauciarticular onset JCA in the families studied and suggest the presence of a disease susceptibility gene or genes within the major histocompatability complex.