The Western world has been used to describing disease on its terms, as if it is a prototype for the same disease found anywhere else in the world. It is unusual that one can test the hypothesis that a common disease can present in markedly different ways, depending on the country in which it is studied. We have had the opportunity to compare and contrast primary hyperparathyroidism in the United States and in China as seen in New York City and in Beijing. The cohort of subjects in each case was well over 100, and the experience extends to well over a decade of observations. In the United States, primary hyperparathyroidism typically presents as asymptomatic hypercalcemia in women within 10 years of menopause. Most often, it is discovered accidentally in the course of a routine multichannel chemistry screening test. The serum calcium is 10.5 + 0.1 mg/dL, within 1 mg/dL above the upper limit of normal, 10.2; the serum parathyroid hormone level is 118 + 9 pg/mL (within 1.5-2-fold above the normal limit, 65). The average 25-hydroxyvitamin D level is 21 ng/mL, in the lower range of normal. The classical clinical manifestations of primary hyperparathyroidism, stone and bone disease, have become much less common than earlier descriptions of the disease in the United States through the 1950s. Overt radiological bone disease (osteitis fibrosa cystica) is almost never seen, whereas stone disease is reduced in incidence from a high of 60% in the 1940s to current estimates of 15-20% now. Most patients are asymptomatic; skeletal involvement is detected only by measuring skeletal calcium by bone densitometry. Primary hyperparathyroidism in China presents much differently. Patients are younger, with an average age of 37. The serum calcium level is much higher, averaging about 12 mg/dL. PTH is over 20 times the upper limits of normal. The average 25-hydroxyvitamin D concentration is much lower than in the United States population, 8.8 ng/mL. Radiological evidence for osteitis fibrosa cystica is seen in 60% of patients; virtually all patients have osteoporosis. Thirty-five percent of patients suffer pathological fractures, most often of the femur or humerus. Forty-two percent demonstrate kidney stones, with half showing bilateral disease. Constitutional features of weakness and easy fatigability are always present. There are both facile and rather subtle explanations for this dramatically different presentation of the same disease in the United States (New York City) and China (Beijing).