A 43-year-old man presented with fluctuating symptoms of weight gain, shortness of breath, pretibial oedema, associated with anxiety and memory disturbances. Laboratory investigation revealed an adrenocorticotropin (ACTH)-dependent cyclical Cushing's syndrome characterised by remarkable variations in urinary cortisol excretions ranging from 27 to 28,050 nmol/ 24 h. Magnetic resonance imaging (MRI ) of the pituitary was normal and ectopic ACTH production was suspected. A tumour in the right anterior mediastinum was revealed on octreotide receptor scintigraphy, which had initially been overlooked on computed tomography (CT) scanning. A thymic carcinoid tumour was suspected, which was supported by increased levels of urinary serotonin, while platelet serotonin and urinary 5-hydroxyindoleacetic acid levels were normal. The tumour was removed surgically and histological examination revealed an atypical thymic carcinoid tumour. Postoperatively, the patient's symptoms disappeared rapidly. He underwent external radiotherapy and is still free of symptoms after almost two years of follow-up. For clinical practice, a cyclical Cushing's syndrome should be suspected in any patient with clinical signs of Cushing's syndrome but normal biochemistry. Repeated measurement of urinary cortisol excretion is then required to establish or rule out the diagnosis.