OBJECTIVE Endogenous carbon monoxide (CO) is produced mainly by heme catabolism. As CO is excreted solely by the lung, a simple technique for measuring the end tidal carbon monoxide (ETCO) level was assessed as a method for screening for haemolytic disease in children. METHODS Two end expiratory breath samples were collected from normal children and from children with haemolytic disease using a one way valve connector between a mouth piece and an anaesthetic bag. The samples were analysed by gas chromatography for CO and carbon for dioxide (CO2). The CO2 value was used to normalise the CO value to an alveolar concentration. Carboxyhaemoglobin (HbCO) also was measured in the patient group for correlation analysis with ETCO. RESULTS A total of 21 children with beta thalassaemia major, 15 children with other haemolytic diseases (hereditary spherocytosis n=8, haemoglobin H disease n=3 and thalassaemia intermedia n=4) and 23 normal children were studied. The mean ETCO concentrations in the three groups were 3.21 p.p.m., 7.41 p.p.m. and 0.69 p.p.m., respectively, which were significantly different from each other (P<0.0001). There was a significant correlation between ETCO and HbCO in the patient groups (r=0.85; P<0.0001). CONCLUSIONS The end expiratory breath collection device is a simple and feasible sample collection method. The results confirm that ETCO can be used clinically to distinguish children with a variety of haemolytic disorders from normal subjects.