Units for the investigation of susceptibility to malignant hyperthermia (MH) were set up in Denmark in 1977 and in Sweden in 1981. Two hundred and ten patients from 76 families have been investigated. The diagnosis of MH susceptibility (MHS) was made by in vitro exposure of muscle from vastus medialis to halothane and to caffeine. MHS criteria for the patients in this paper were established from examination of 31 control biopsies, obtained from the same muscle and with the same anaesthesia as the MH patients. The criteria have since been changed to those presented elsewhere in this issue. In our laboratories the halothane test (exposure to 0.5-2% halothane) was the more sensitive: 88% of MHS patients reacted to it. The caffeine test was positive in 68% of MHS patients, 0.5-2.0 mmol litre-1 solutions being the most discriminating. Forty-two percent of MHS patients reacted to only one test. Fulminant MH was the most common reason for investigation; all these families contained MHS members. Masseter spasm occurred as sole sign in 21 families, of which 11 were MHS. Only 10% of MHS patients had other signs or symptoms of neuromuscular disease such as muscle cramps or muscular dystrophy. Three families had experienced sudden infant death syndrome (SIDS), and two teenage brothers in a MHS family died suddenly, but death was unrelated to anaesthesia.