Five cases of infarction of physiological hyperplastic breast tissue associated with pregnancy or lactation are reported. These presented clinically as lumps in the breast, two of which were painful. Similar cases in the literature are briefly reviewed. The demonstration of an underlying lobular pattern in the present cases and the distribution of the infarcts support the view that the so-called 'adenoma' of pregnancy or lactation is a myth. Organizing vascular thrombi, previously unrecorded in this context, were seen in three of the five cases, and it is postulated that these lesions are the primary cause of the infarction rather than relative vascular insufficiency, as proposed by previous authors.