Background:The German Pegvisomant Observational Study (GPOS) was created immediately after marketing authorisation was received in Germany for Somavert®(pegvisomant) for the treatment of patients with acromegaly. In August 2006, the database underwent its fifth interim analysis of 263 patients, the vast majority of whom previously had insufficient disease control with other treatment modalities. The GPOS documents both safety and efficacy aspects of the treatment of patients with acromegaly by the first growth hormone-receptor antagonist, pegvisomant. This treatment led to normalization of disease activity in most patients, had favourable effects on glucose metabolism and improved signs and symptoms of the disorder. The safety profile indicates that pegvisomant treatment is well tolerated, and tumour growth is noted to occur at the same rate as for somatostatin analogue treatment. Transaminase elevations occurred in 16 of the 263 patients but spontaneously resolved in eight of them and promptly normalised in five patients who discontinued treatment. GPOS is presently the largest database of pegvisomant-treated patients, and it comprises more than 87% of all patients treated with pegvisomant in Germany. Conclusions: The GPOS database provides important information about treatment modalities, safety and efficacy of pegvisomant in patients with acromegaly.