Height, weight, and skinfold thickness were measured on 82 patients prior to muscle biopsy which was performed to determine their susceptibility to malignant hyperpyrexia. Percentage body fat was calculated from the skinfold measurements. Using AP photographs six coronal diameters of the left thigh, equally spaced between the lower border of the patella and the perineum, were measured on another group of 90 patients referred for biopsy. The subjects were then divided into those who were susceptible to malignant hyperpyrexia (MHS) and those who were normal and acted as controls. Each group was separated into males and females. There was no significant difference in age between the MHS and the control groups. There was no difference between MHS and controls in height or weight but the percentage body fat in the MHS males was significantly lower than in the controls (P less than 0.02). The upper three thigh diameters in the MHS females were significantly greater than in the control group (P less than 0.05). There appears to be a difference in the leanness/fatness relationship and in the development of the thigh in MHS subjects compared with controls, but these differences are subtle and appear to vary with sex.