OBJECTIVE--To examine whether use of the active compression-decompression device improves the haemodynamics of cardiopulmonary resuscitation compared with those of conventional cardiopulmonary resuscitation. DESIGN--Prospective crossover study. SETTING--The accident and emergency department of a university teaching hospital. PATIENTS--36 adult patients with non-traumatic, normothermic, out of hospital cardiac arrest. INTERVENTIONS--Cardiopulmonary resuscitation was performed during resuscitation in alternating 3 min cycles of conventional and active compression-decompression cardiopulmonary resuscitation. MAIN OUTCOME MEASURES--The end tidal carbon dioxide (ETCO2), femoral arterial pressure, and acid-base analysis of central venous blood measured during the last 30 s of each 3 minute cardiopulmonary resuscitation cycle. RESULTS--ETCO2 was monitored in 36 patients during conventional and active compression-decompression cardiopulmonary resuscitation. Active compression-decompression cardiopulmonary resuscitation caused a significant increase in ETCO2 (P < 0.0002), indicating improved cardiac output. Arterial pressure measurement was carried out in 10 patients. Systolic pressure was significantly greater with active compression-decompression than conventional cardiopulmonary resuscitation (P < 0.007). Central venous blood was taken for acid-base analysis in 11 patients. There was a significant increase in the central venous hydrogen ion concentration (P = 0.025) with rises in the partial pressures of carbon dioxide and oxygen, suggesting improved venous return. CONCLUSIONS--This study confirms that active compression-decompression cardiopulmonary resuscitation is associated with better haemodynamic status than conventional resuscitation.