Malignant neoplasms were found at 194 of 723 autopsies carried out at the Marqués de Valdecilla National Medical Center in the period 1980-1982, and in 50 of these there was secondary involvement of skeletal muscle. In 16 cases (8.2%) the primary tumour and/or its metastases had invaded skeletal muscle, but in 34 (17.5%) true macroscopic or microscopic metastases were present. It was this latter finding which prompted us to carry out a detailed analysis. Below are given the percentages of malignant neoplasms of each type in which the tumours had metastasized to skeletal muscle and, in brackets, the number of metastases found in skeletal muscle as a percentage of all metastatic tumours. Acute leukaemias 17.2% (14.7%), lymphomas 20.6% (17.6%), carcinomas 17.7% (64.6%): for the various sub-categories of carcinoma the figures were: epidermoid carcinoma 10% (5.8%), adenocarcinoma 29.6% (47%) and anaplastic carcinoma 5.8% (2.9%). The muscles most commonly involved were the diaphragm (67.6%) and the iliopsoas (29.4%). The patients ranged in age from 26 to 84 years, mean age 62.8 years. The male/female ratio was 2.8/1. In advanced cancer an appreciable percentage of metastases occur in skeletal muscle.