Plasma and urine phosphate concentrations were improved in 21 patients with diabetes mellitus during "optimal" metabolic control as compared with "suboptimal" control. During the "suboptimal" control phase the daily insulin dosage averaged 38 +/- 22 (SD) U/day and the mean plasma glucose levels averaged 17.1 +/- 1.8 mmol/l, while during the "optimal" control phase the daily insulin dosage averaged 84 +/- 59 U/day and the mean plasma glucose level was 6.2 +/- 1.4 mmol/l. The institution of rigid diabetic control over 4-10 days significantly raised serum phosphorus from 1.12 +/- 0.16 to 1.26 +/- 0.19 mmol/l (p less than 0.001), and decreased urinary phosphorus excretion from 686 +/- 125 to 588 +/- 88 mg/day (p less than 0.001). These changes were associated with significant reductions in urinary calcium, urinary glucose, plasma immunoreactive glucagon and serum parathyroid hormone. This diminution in urinary phosphorus loss may have been due to diminished glycosuria but equally could have been influenced by a direct action of insulin on the renal tubule or suppression of glucagon and parathyroid hormone secretion. Under the conditions of this study, reduced urinary phosphorus may have been sufficient to cause a rise in serum phosphorus despite the known effects of insulin on the cellular influx of phosphorus.