The annual incidence of primary hyperparathyroidism (PHP) is considered about 20 cases per 100 000 and PHP is the third most common endocrine disease after diabetes mellitus and thyroid dysfunction. In modern medicine the clinical spectrum of PHP has changed during recent decades – based on the routine screening of serum calcium most cases are diagnosed in an asymptomatic stage. In Estonia the routine screening of serum calcium has not been established and many patients present with classical manifestations of PHP. We report four cases of classical PHP with different disease manifestations: osteitis fibrosa cystica with a typical brown tumour, chronic pancreatitis and skeletal manifestations, pepticulcer and renal dysfunction, and neuromuscular syndrome. All patients underwent successful surgical treatment. However, after resolving PHP by surgery one of the patients with bone disease manifested a severe “hungry bone” syndrome which required treatment with unusually high doses of calcium carbonate and calcitriol. In the discussion both classical and modern PHP are reviewed and the conditions in which calcium assay is strongly requested are proposed.