Maternity Blues, although seldom a serious problem in clinical practice, is potentially important to research on affective disorders in general. Childbirth is a major life event known to be associated with large changes in maternal hormones. The determinants of the Blues may therefore be psychological and social, or biological, or both. This paper reviews the relevant literature. Reported associations between Maternity Blues and psychiatric disorder are examined. Possible psychological, social and biochemical determinants are reviewed, but no firm inferences on causation can yet be drawn. Conflicting results in the literature may have been due to variations in definition and measurement of the syndrome. The authors have recently used psychometric methods to develop a questionnaire for detecting and measuring Maternity Blues. By cluster analysis of responses to the questionnaire, a 'Primary Blues' cluster was defined, consisting of 7 items: tearful, tired, anxious, over-emotional, up and down in mood, low spirited, muddled in thinking. The item 'depression' appeared in another less frequent cluster.