As a general principle in the treatment of primary hyperparathyroidism due to single adenoma, unilateral parathyroidectomy was applied to 50 patients and compared with another group of 50 conventionally explored patients. Twenty-five patients were explored only on the "adenoma" side. The other 25 patients were explored on both sides, avoiding biopsies at the first. In the conventionally explored patients, the adenoma was removed and one to three normal glands were biopsied. Oil-red-O technique was used in the intraoperative microscopical examination. The patients in whom the operation could be limited to the "adenoma" side had a statistically more favorable situation concerning early postoperative hypocalcemia, length of operation time, and need for calcium and vitamin D substitution. The principle of unilateral parathyroidectomy in conjunction with intraoperative oil-red-O staining technique is advocated in hyperparathyroidism due to single adenoma because it offers more reliable peroperative distinction between uni- and multi-glandular involvement, reduced operation time, decreased risk for complication, reduced early hypocalcemia, and more favorable technical conditions for reoperation.