Axillary surgery for breast cancer partially obstructs lymph outflow from the arm, chronically raising the lymphatic smooth muscle afterload. This may lead to pump failure, as in hypertensive cardiac failure, and could explain features of breast cancer treatment‐related lymphoedema (BCRL) such as its delayed onset. A new method was developed to measure human lymphatic contractility non‐invasively and test the hypothesis of contractile impairment. ^99mTc‐human IgG (Tc‐HIG), injected into the hand dermis, drained into the arm lymphatic system which was imaged using a gamma‐camera. Lymph transit time from hand to axilla, t_transit, was 9.6 ± 7.2 min (mean ±s.d.) (velocity 8.9 cm min⁻¹) in seven normal subjects. To assess lymphatic contractility, a sphygmomanometer cuff around the upper arm was inflated to 60 mmHg (P_cuff) before ^99mTc‐HIG injection and maintained for >> t_transit. When P_cuff exceeded the maximum pressure generated by the lymphatic pump (P_pump), radiolabelled lymph was held up at the distal cuff border. P_cuff was then lowered in 10 mmHg steps until ^99mTc‐HIG began to flow under the cuff to the axilla, indicating P_pump≥P_cuff. In 16 normal subjects P_pump was 39 ± 14 mmHg. P_pump was 38% lower in 16 women with BCRL, namely 24 ± 19 mmHg (P= 0.014, Student's unpaired t test), and correlated negatively with the degree of swelling (12–56%). Blood radiolabel accumulation proved an unreliable measure of lymphatic pump function. Lymphatic congestion lymphoscintigraphy thus provided a quantitative measure of human lymphatic contractility without surgical cut‐down, and the results supported the hypothesis of lymphatic pump failure in BCRL.