The dynamic changes in serum phosphorus levels in 69 episodes of ketoacidosis in 48 diabetic patients were retrospectively evaluated. The mean age was 41 +/- 2 years (mean +/- SEM), and the duration of diabetes mellitus was 7 +/- 1 years. The serum phosphorus levels determined within the first six hours of admission were analyzed. Before initiation of therapy, the incidence of hyperphosphatemia was 94.7 percent. At the end of 12 hours, the mean serum phosphorus level fell from 9.2 +/- 0.6 to 2.8 +/- 0.3 mg/dl. Before therapy, the serum phosphorus level correlated positively with the serum glucose level, the effective plasma osmolality, and anion gaps, and correlated negatively with the serum chloride level. It is concluded that hyperphosphatemia is common in diabetic ketoacidosis before therapy. The increase in serum phosphorus is likely to be due to a transcellular shift. Potential factors responsible for the shift are serum glucose, through its osmotic effect, and the organic anions.