Much the most common type of seasonal affective disorder at temperate latitudes is recurrent winter depression, which probably affects around 3% of adults in the UK to a clinically significant degree. In this article, diagnosis and presentation are discussed and symptoms are contrasted with those of nonseasonal depression. Aetiology and epidemiology, with regard to age, gender and latitude of residence, are described. Sufferers are often treated with light therapy, and this is described in some detail, with mention of effectiveness, prediction of outcome, timescales of response, side-effects, use of lightboxes and alternatives to lightboxes. Other general aspects of the management of seasonal affective disorder, including the use of antidepressant medication, are also outlined.John Eagles is a consultant psychiatrist at Royal Cornhill Hospital (Cornhill Road, Aberdeen AB25 2ZH, UK) and an honorary reader in mental health at the University of Aberdeen. He has been assessing and treating patients with seasonal affective disorder for 10 years. His other research interests include eating disorders and suicide.