This paper reviews current knowledge and hypotheses about the physiology of lymph production and lymph flow and the aetiology and classification of lymphoedema. One of the earlier contributions in the 1930's by Allen suggested that primary lymphoedema was caused by congenital underdevelopment of lymph vessels. At that time he described two clinical varieties (congenital and praecox) but later suggested a further subdivision into inflammatory and non-inflammatory types of lymphoedema. Kinmonth in 1957 produced the first clinical classification dividing all cases into primary or secondary lymphoedema but at that time gave no indication as to the cause of the primary variety. In the 1950's Kinmonth also developed a radiological classification which has contributed considerably to our knowledge of the anatomical abnormalities of the lymphatic system in primary lymphoedema. However, despite the widespread use of Kinmonth's classification the lymphographic appearances in primary lymphoedema give little or no clue to the aetiology of the disease process. The aetiology of primary lymphoedema is not known. There are various descriptive classifications based on age of onset and radiological findings for instance, but none attempts to explain the pathophysiology of the disease. We have therefore described our current knowledge of the known physiology and the potential abnormalities of the collection and passage of lymph from the interstitial space to the blood system. Based on this we have presented a simple classification of the aetiology of lymphoedema. Thus primary lymphoedema may be defined as lymphoedema caused by a primary abnormality or disease of the lymph conducting elements of the lymph vessels or lymph nodes. Secondary lymphoedema is oedema caused by disease in the nodes or vessels that began elsewhere (e.g., neoplasia or filariasis), or lymphocytic proliferative disorders such as Hodgkin's disease or following surgical extirpation of lymph nodes or vessels. There are three groups of primary lymphoedema in which the functional abnormality and its cause are known; namely (a) large vessel abnormalities such as congenital aplasia of the thoracic duct or cysterna chyli, (b) congenital lymphatic valvular incompetence or congenital aplasia and (c) lymph node fibrosis. The remainder are characterised by a reduced number of lymphatics on lymphography. Such patients can be described as having obliterated lymphatics. If they present within a few years of birth they were probably born with too few lymphatics. However, those who present later in life may have acquired obliterative disease, the cause of which is still obscure.(ABSTRACT TRUNCATED AT 400 WORDS)