The measurement of urinary or nephrogenous cAMP is considered to be a reliable index of parathyroid function. This test usually involves timed urine collections which renders it logistically burdensome. We have therefore assessed the validity of using spot urine samples for this purpose. 2-hour urine collections were compared to spot samples for the estimation of urinary and nephrogeneous cAMP in 10 patients with hypercalcemic primary hyperparathyroidism and 22 control subjects. The results demonstrated: (1) a considerable overlap between the primary hyperparathyroid and control groups when the urinary and nephrogenous cAMP values were expressed as a function of creatinine excretion. (2) A satisfactory separation between the two groups when the 2-hour urine and spot sample levels of urinary cAMP – and even more so nephrogenous cAMP – were expressed as a function of glomerular filtrate (i.e. nmol cAMP/100 ml glomerular filtrate). (3) An excellent correlation (r = 0.91, p < 0.001, n = 32) between 2-hour urine collections and spot samples for both urinary and nephrogenous cAMP levels. The feasibility of using a spot sample for measuring urinary or nephrogenous cAMP without impairing its diagnostic efficacy considerably simplifies the logistics of the test. This is of particular importance for ambulatory patients.