A previously fit 39 year old joiner presented in November 2003 with a 6 month history of intermittent right-sided abdominal pain. There was no associated nausea or vomiting. An abdominal ultrasound requested by his general practitioner revealed a single gallbladder calculus and a 10 cm right-sided cystic mass with some echogenic areas, probably of hepatic or adrenal origin. He was normotensive and clinical examination was normal. CT scan of the abdomen confirmed a 9 cm mass arising from the right adrenal gland with low attenuation. Full blood count, renal and liver functions were normal. 24-hour urinary cortisol and catecholamines were normal. Serum testosterone, androstenodione, DHEA-S, LH and FSH levels were normal. Anti-phospholipid antibodies were negative. Serology for hydatid and amoebae were negative. Tumour markers were normal. A MIBG study showed that the mass was MIBG-negative. Repeat ultrasound in December 2003 showed a reduction in size of the mass to 5 cm with no cystic components and a CT abdomen in August 2004 showed a well-defined homogenous mass with further reduction in the size to 4 cm. In view of the reduction in size of the mass, he was kept under review over the next year. Although he reported worsening right-sided abdominal pain, a repeat CT scan in November 2005 showed further reduction in size to 3 cm. However, in view of the continuing pain, he underwent a laparoscopic right adrenalectomy. Histology of the mass revealed an adrenal pseudocyst with no evidence of malignancy. To our knowledge, this is the first reported case of a shrinking adrenal pseudocyst. Adrenal pseudocysts are rare and usually present with symptoms related to tumour size. If a substantial cystic component to an adrenal mass is seen, then this diagnosis should be considered. The normal size-related criteria for malignancy in adrenal masses is not appropriate in this circumstance.