Aim: To present the experience of the UK Pulmonary Hypertensive Service for Children at Great Ormond Street Hospital in treating children with “Eisenmenger” type physiology with the dual endothelin receptor antagonist bosentan. Patients and methods: Fourteen inoperable patients were included in the study. These were selected based on a history of an unrestrictive intracardiac communication and established right to left shunt at rest. There was a female: male ratio 1: 0.75 and age range at initiation of treatment of 7–18.6 years (mean 12.5 yrs). Six patients had Trisomy 21. The cardiac diagnoses were: 6 large VSDs, 6 CAVSD (1 of which was not Trisomy 21) and 2 more complex congenital heart disease cases that underwent palliative surgery (DILV, DOLV, TGA, PA banding and DORV, TGA, VSD, PS, Waterston). Twelve patients received antiplatelet/anticoagulation therapy. WHO functional class, haemoglobin (Hb), oxygen saturation, 6 min walk tests, echocardiograms and ECGs were assessed before starting treatment with bosentan and after 6 and 12 months of follow up. Twelve patients had a cardiac catheterisation before treatment commenced. The parameters were compared for patients before treatment and at 6 and 12 months post treatment with paired t-test and ANOVA. Results: All patients were severely symptomatic the mean WHO functional class being 3.2 (range 3–4). The mean Hb was 18.3 g/dl (range 13.4–22.9). The mean PVR of all patients was 30.98 units m2 (7.6–57). The Trisomy 21 patients had a mean PVR of 34.4 units m2 and non Trisomy 21 patients had a mean of 28.6 units m2. Mean oxygen saturation at rest was 78.9% (range 67–93), mean 6 min walk test distance was 223.8 m (n = 10, range 108–360) and the lowest oxygen saturation during the walk test was 59.7% (42–79) at 6 min or below. Mean duration of therapy was 15.2 months (range 6–34 months). Bosentan was well tolerated in all patients but discontinued in one due to non compliance with monthly screening blood tests. All patients remained stable and their echocardiogram and ECG findings were unchanged throughout. After 6 months mean WHO functional class had decreased significantly to 2.96 (p = 0.029), whilst Hb and oxygen saturation at rest had not changed. Ten of the 14 patients were on treatment for more than a year and after 12 months the WHO functional class was maintained at the same improved level. The Hb and oxygen saturation at rest remained unchanged. After 12 months the 6 min walk mean distance had increased from 211 to 253 metres (non significant). Conclusion: Patients treated with bosentan for up to 1 year showed clinical improvement and remained stable. After 6 months treatment WHO functional class had significantly improved and improvement was sustained for up to 1 year.