A 36-year-old man with sarcoidosis had four episodes of hypercalcaemia in seven years, all of them during the summer months. Measurement over three years showed that hypercalcaemia was associated with small seasonal increases in serum-25-hydroxycholecalciferol within the normal range. These changes could be mimicked by the administration of 3000 units of vitamin D3 daily. Serum 1, 25-dihydroxycholecalciferol concentrations ranged between 26--62 pg/ml when serum calcium was normal, but were strikingly high, up to 137 pg/ml, when the patient was hypercalcaemic. These studies show for the first time that hypercalcaemia in sarcoidosis is associated with abnormally high circulating concentrations of 1, 25-dihydroxycholecalciferol, probably as a result of overproduction of this, the hormonal form of vitamin D.