Survivors of acute lymphoblastic leukaemia (ALL) are recognised to become overweight and this seems to worsen with increasing length of follow up. Increases in body fat appear to be more marked in girls than in boys and in those who have received prophylactic cranial irradiation. Physiological responses to exercise, both at submaximal and maximal levels, are different in ALL survivors compared to controls. Heart rate appears to be increased at low intensity exercise, possibly to maintain adequate cardiac output. Maximal aerobic capacity is reduced, signifying a lower level of physical fitness. Total daily energy expenditure (TDEE) under free living conditions appears limited due to low participation in physical activity. Associations exist between measures of energy expenditure and body fat, but whether these are cause or effect has yet to proven. Pediatr Blood Cancer 2008;50:456–461. © 2007 Wiley-Liss, Inc.