Objectives: Sleep apnoea has been consistently reported to occur in acromegaly. In uncontrolled patients, the severity of sleep apnoea influences physical activity in the daytime. We investigated the influence of disease activity on tongue volume and sleep apnoea treated with the GH receptor antagonist pegvisomant in poorly controlled patients with acromegaly under octreotide. Design and methods: A total of 12 patients with active acromegaly (six females; six males; mean age 57G15 years; body mass index 29.4G4.2 kg/m 2 ; meanGS.D.) were treated with pegvisomant (13.5G5.0 mg/die) for 6 months. Tongue volume was examined by magnetic resonance imaging, and sleep apnoea was characterized by polysomnography before and after 6 months of treatment with pegvisomant. The mandibular length was determined by lateral X-ray films. Results: IGF1 levels decreased after 6 months in all patients (407G114 to 199G23 mg/l; PZ0.0001). The tongue volume decreased (105G33 to 83G33 ml; PZ0.007) as well as the apnoea-hypnoea index (23G22 to 18G18/h; PZ0.0066). The mandibular length correlated with the initial tongue volume (r 2 Z0.6072, PZ0.0028). Conclusion: In conclusion, successful treatment with pegvisomant can decrease tongue volume, which has benefits for coexisting sleep disordered breathing.