Obesity seems to be a common comorbidity in patients with pandemic influenza H1N1 infection who are admitted to hospital, 1 need intensive care treatment, 2 or who have died. 3 Until recently, obesity has not been thought to be a major factor in the outcome of infectious diseases. Why this association between obesity and the outcome of influenza infections is emerging is unclear. The increasing prevalence of obesity within a population might reveal an association not evident at lower prevalences; perhaps, as with diabetes and tuberculosis, obesity increases the risk of and affects disease presentation. 4 Previous cohort studies of patients with bacterial infections suggest that obesity alters the course of infection; 5, 6 and obesity seems to contribute to the progression of some chronic viral infections such as hepatitis C infection. 7 Several factors limit opportunities to research the relation between obesity and clinical infectious diseases. In Finland, for example, body mass index (BMI) data are not routinely recorded during patients’ hospital visits, which limits the possibility for retrospective studies and investigations of registry data to assess the role of obesity in patients admitted to hospital with infectious diseases. This limitation could be overcome if weight and height measurements were routinely recorded on hospital admission. However, these data would not be recorded for most critically ill patients. BMI data were lacking for many patients in two recent studies of H1N1 in the USA; where data were available, how they had been collected (objective measurement or questioning of patients) was unclear. 1, 3 Other investigations of the effect of obesity on outcomes of critical illness have identified similar limitations. 8 But perhaps these substantial limitations would be acceptable in investigations of a disease for which epidemiological insights are eagerly awaited, as with H1N1. Nonetheless, some researchers have questioned the usefulness of BMI data in critical care, since the most severely ill patients need the largest volume of fluids for fluid therapy, which raises patients’ total measured body weights and their BMI. 9