The musculovenous pump function was assessed by ambulatory strain gauge plethysmography in 11 patients with primary lymphoedema. The venous function was within the normal range regarding both venous reflux and expelled volume. The values were compared with ten patients with chronic venous insufficiency and oedema. The values for venous function were clearly abnormal for the patients with venous insufficiency and different from those with primary lymphoedema. Ambulatory strain gauge plethysmography may be a useful non-invasive method in distinguishing between oedema of lymphatic or venous origin.