It is generally accepted that some patients affected by mild asymptomatic primary hyperparathyroidism need not be treated with surgery, but may be medically managed without risk. However, our experience regarding 5 of these cases observed in the last two years, suggests a different approach. These patients, initially diagnosed as having mild hyperparathyroidism based on only moderately elevated serum concentrations of calcium and followed medically for years, were referred to us for a sudden worsening of their clinical course. One 35-year-old man presented hemorrhagic gastritis with severe anemia and type II AV block with syncopal attacks. Three women, aged 51, 64 and 65 years, presented with severe hypercalcemia associated with renal failure in two and with marked bone disease in another. In all these cases parathyroid neoplasms were preoperatively localized (by ultrasonography, CT scan and radioactive 201-Tl 99-Tc scan) and surgically removed. Histological examination showed a parathyroid carcinoma in the male patient and single gland enlargements in the three females. A fifth patient, a 65-year-old woman, was referred to us in critical condition: severe hypercalcemia, osteopenia with femur fracture, myocardial infarction and renal failure. She died in a few days, in spite of intensive medical care. These cases suggest that patients with hyperparathyroidism initially diagnosed as "mild" need close medical observation and preferably, in our opinion, should undergo surgery.