Excised muscles from patients with myotonia or periodic paralysis were subjected to the in vitro contracture test for susceptibility to malignant hyperthermia (MH). In a group of 44 patients, this standard test gave four positive, 10 equivocal and 30 negative results. The results for 27 control muscles from normal subjects were negative. When the test was performed with less than normal concentrations of contracture-triggering substances (caffeine < 2 mmol litre−1, 2+] in myotonic dystrophy. This shows that the in vitro contracture test lacks specificity.