Analysis of sex ratio in 301 siblings of 150 patients with early-onset pauciarticular juvenile rheumatoid arthritis revealed a male-to-female ratio of 1:2.00 in sibships with an HLA-B44+ proband, compared with a ratio of 1:1.05 in other sibships (G2 = 6.07, df = 1, p = 0.014). The siblings had a sex ratio of 1:0.8, when the HLA-B44 antigen was present in either parent but not transmitted to the proband. The capacity to distort the sex ratio was limited therefore to disease-associated HLA-B44 haplotypes.