Antinuclear antibodies (ANA), as detected by indirect immunofluorescence on HEp-2 cells, have been investigated in five spouses and 41 first-degree relatives of nine probands with polymyositis-dermatomyositis (PM-DM) and in 41 sex- and age-matched controls. ANA were detected in 12 out of the 41 first-degree relatives and in two controls (chi 2 = 6.97; P less than 0.01). HLA typing was done in four out of the nine families; in two of them only, ANA segregated with a haplotype. ANA positivity was not correlated either to sex or to age or to household contact. Our results show that ANA occur in a significant percentage of first-degree relatives of patients with PM-DM. The finding seems to be genetically conditioned.