The report by Åberg et al ^{Reference Åberg, Waern, Nyberg, Pedersen, Bergh and Åberg1} of an association between lower cardiovascular fitness at age 18 and serious depression in adulthood is interesting but their conclusion that the results ‘strengthen the theory of a cardiovascular contribution to the aetiology of depression’ is questionable. The authors recognise that although they controlled for psychiatric symptoms and disorders prior to and at the time of baseline assessment of cardiovascular fitness, they did not obtain information ‘which could help in defining subsyndromal affective problems’, nor details of ‘other possible confounders such as personality, smoking and low self-esteem’ that may increase risk for both poor fitness and depression. Seemingly lower cardiovascular fitness is not a risk factor for bipolar depression (or mania) and may not be a risk factor for females. ^{Reference Sund, Larsson and Wichstrom2} It is not stated whether consideration was given to psychiatric disorders apart from depression (e.g. anxiety disorders) and this leaves uncertainty as to whether lower cardiovascular fitness at age 18 is only a risk factor for non-bipolar depression in males. Considering that all study participants were sufficiently fit for recruitment into national service, it would be difficult to explain how a mere difference in cardiovascular fitness could (as suggested by the authors) contribute to cause ‘severe’ depression even before the age of 30. Given a recognised relationship between adverse developmental experiences, personality traits and depression, ^{Reference Heim, Newport, Mletzko, Miller and Nemeroff3,Reference Boyce and Mason4} it would be important to obtain information about such potentially confounding variables as well as ‘subsyndromal problems’ at the time of assessing cardiovascular fitness. It is suggested that in light of such missing information, the reported finding does not as yet ‘strengthen the theory of a cardiovascular contribution to the aetiology of depression’. It is more an interesting association that warrants further investigation.