Carbon dioxide (CO2) is the gas most commonly used to inflate the body cavities during 'keyhole' surgery (e.g. laparoscopy and thoracoscopy). However, CO2 can be absorbed, leading to increased arterial CO 2 and increased CO2 elimination from the lungs. These increases in CO2 are observed following a wide variety of procedures both in adults and in infants and children. Although it is usually assumed that increases in arterial or end-tidal CO2 directly reflect absorption of CO2 from body cavities, this is not necessarily true, as either increases in metabolically produced CO2 or respiratory compromise making it more difficult to eliminate CO2 could also be responsible for these changes. Recently, a new technique has been introduced which enables absorbed CO2 to be distinguished from metabolic CO2.