A man aged 59 years developed a rapidly progressive muscular weakness which within a few weeks involved almost the entire musculature. A carcinoma of the stomach was discovered at necropsy. The skeletal muscles showed widespread necrosis with minimal inflammatory reaction. The findings are compared with those in a woman of 51 with Hashimoto's thyroiditis who developed a slowly progressive weakness of her trunk muscles which after some months began to involve the limbs. Post-mortem examination revealed typical polymyositis with massive inflammatory infiltration and limited necrosis of muscle. We conclude that these two types of muscle disease are morphologically, and probably pathogenetically, different.