Calcium and phosphous metabolism was investigated in 20 patients with diabetes mellitus when their diabetes was under poor metabolic control and again once optimal glycaemic control was achieved with aggressive insulin therapy. Ten of the twenty uncontrolled diabetics had hypercalciuria; insulin therapy returned calcium excretion to normal in five. Twenty-four hour calcium excretion fell in all but two patients when optimal diabetic control was achieved and calcium excretion was positively correlated with glucose excretion. Urinary cyclic AMP excretion, which was in the high normal range during poor control, decreased significantly during optimal insulin therapy. These data suggest that the hypercalciuria of uncontrolled diabetes may be a form of renal hypercalciuria which could result in parathyroid stimulation which might contribute to the development of osteopenia in patients with diabetes mellitus.