Multiple gland enlargement (M.G.E.) in primary hyperparathyroidism (H.P.T.) is the presence of 2 or more enlarged glands weighing more than 50 mg. Conservative surgery (C.S.) consists of resecting the grossly enlarged glands without biopsying the normal glands. Some authors have suggested that C.S. might overlook minute hyperplasia, hence leading to late recurrences of H.P.T., or conversely result in the unnecessary resection of grossly enlarged, but not hyperfunctioning glands. 379 patients have been operated upon for H.P.T. 86 (22.7%) had M.G.E. including 13 (15.2%) M.E.N. cases, 8 (9.3%) familial cases and 65 (75.6%) seemingly sporadic cases (S.S.C.). 2, 3 or 4 glands (or more) were involved in 39.5%, 35% and 25.5% of cases respectively. 1 patient died post-operatively and 3--all S.S.C. with hyperplasia--had to be reoperated upon within 1 year for persistent hypercalcemia. Pathological diagnosis was: double adenomas in 5.8%, hyperplasia in 36%, adenoma associated with hyperplasia in 46.5% and a normal second gland, on light microscopy findings in 11.7%. None of 13 deaths occurring during follow-up was related to H.P.T. 78 patients (90.7%) are available for follow-up (av. 85.3 months). 85.9% are normocalcemic (2.2 less than Ca less than 2.6 mmol/l), 5.1% hypocalcemic and 9% hypercalcemic. 61 had late i. P.T.H. assay; i. P.T.H. was appropriate to serum calcium in 78.7% and appropriate to normal serum calcium levels in 90.6% of 53 normocalcemic cases.