To determine the prevalence of 52-kd and 60-kd Ro/SS-A antibodies in Japanese patients with polymyositis/dermatomyositis, we examined serum samples from 61 patients with PM/DM, 10 patients with primary Sjögren's syndrome, and 25 healthy control subjects. Six serum samples possessed anti-Ro/SS-A antibodies and were positive for anti-Ro52, anti-Ro60, or both. Two reacted with both Ro52 and Ro60, and 4 reacted with Ro52 alone. The results suggest that Ro52 is the main antigen of anti-Ro/SS-A antibodies in patients with polymyositis/dermatomyositis and that its coexistence with other defined antibodies suggests the existence of a subgroup of patients with various serologic abnormalities.