Before 1934 primary hyperparathyroidism was recognized rarely and only in the form of osteitis fibrosa cystica. After Albright demonstrated its occurrence in patients with little or no skeletal disease, with urinary lithiasis, and with minimal evidence of biochemical disorder, it was diagnosed more frequently. In the present series of cases the diagnosis was made in 16 patients in 15 years before and in 364 patients in 18 years after adopting Albright's recommendations. Of these 380 patients, 42 had classic osteitis and 259 had no clinical evidence of bone disease; 291 had symptoms of urinary calcinosis and 89 did not. Some had symptoms of hypercalcemia, including weakness, bradycardia, and polydipsia, but often laboratory findings alone suggested the diagnosis. Hypercalcemia has thus far been the one finding essential to the diagnosis; although sometimes minimal, it has always been present.