The response to exogenous parathyroid hormone (PTH) with urinary excretion of phosphate and cyclic adenosine monophosphate (cAMP) was tested by the use of synthetic human parathyroid hormone (1-34) [hPTH-(1-34)] on 59 patients with hypocalcemia and normal or high serum inorganic phosphorus and normal renal function without a history of parathyroidectomy for differentiation between idiopathic hypoparathyroidism (IHP), pseudohypoparathyroidism (PHP) and related diseases along with 18 normal subjects. A positive phosphaturic response to exogenous PTH was defined as the increment of 2 hours phosphate excretion (delta P) of more than 35 mg. A positive urinary cAMP response to exogenous PTH was defined as the increment by more than 1 mumole per one hour (delta cAMP) and the increase of 1 hour excretion by more than 10 times. Increments of 2 hours urinary phosphate excretion in response to hPTH-(1-34) 100 units were 60.5 +/- 7.7 mg (mean +/- SEM) in 27 patients with IHP, 23.5 +/- 5.9 mg in 21 patients with PHP type I and 24.9 +/- 4.0 mg in 17 normal subjects. Increments of 1 hour urinary cAMP excretion in response to hPTH-(1-34) 100 units were 12.0 +/- 1.5 mumole in 27 patients with IHP, 0.33 +/- 0.10 mumole in patients with PHP type I and 23.6 +/- 5.8 mumole in 15 normal subjects. Ratios of 1 hour urinary cAMP excretion were 97 +/- 10 in 27 patients with IHP, 3.6 +/- 0.5 in 21 patients with PHP type I and 54 +/- 14 in 15 normal subjects. Positive phosphaturic and negative urinary cAMP response was encountered in 3 out of 21 patients with PHP type I in response to hPTH-(1-34). This exaggerated phosphaturic response should be considered as due to the influence of treatment with Ca or vitamin D derivatives.