In a study of 144 women, mainly self-designated PMS sufferers, the premenstrual depression experienced was, apart from its shorter duration, quantitatively and qualitatively similar to major depressive disorder for a substantial proportion of subjects. The associations with previous history of depression were complex: the severity of premenstrual depression was related to previous history of postnatal depression, whereas its duration (i.e., whether it persisted through longer) was related to a history of treatment with antidepressants. Two independent dimensions are proposed. (i) A menstrual cycle-related factor which in vulnerable women can results in severe and disabling premenstrual dysphoria, and which may be aetiologically related to a subgroup of postnatal depression. (ii) In a minority of women a more general propensity for depressive illness evidence as a tendency for any premenstrual depression to be prolonged.