Introduction. Patients with primary hyperparathyroidism (pHPT) run an increased risk of death, and in some studies cardiovascular diseases were inversely related to glomerular filtration rate (GFR) and urine osmolality. Aim — to evaluate the renal filtration function and concentration capacity in patients with mild primary hyperparathyroidism. Material and methods. The study included 100 patients with pHPT (median age 57 (52; 61), including 33 with mild form (median age 54 (45; 60). Changes in GFR and osmolality index were evaluated in 29 patients after surgery for pHPT. Follow-up period was up to 24 months. Osmolality index was calculated as urine osmolality to blood osmolality ratio. Renal concentration capacity impairment was diagnosed with osmolality index less than 2. Glomerular filtration rate was calculated by Modification of Diet in Renal Disease Study (MDRD) formula. Chronic kidney disease stage was estimated accordingly to current recommendations. Results. Osmolality index in patients with mild pHPT was low with median 1.65 (1.4; 2.43). We found a high prevalence of renal concentration capacity impairment in patients with mild pHPT, that was 70%. Mean GFR was 90.9 (73.3; 95.6) ml/min/1,73 m2. Prevalence of chronic kidney disease stages 3—4 was 6% in patients with mild pHPT. Changes in renal concentration capacity in long-term period after surgery for pHPT were characterized by increase of osmolality index, also in patients with mild form (initially 1.75 (1.4; 2.14), after surgery 2.38 (1.84; 2.54), changing Me was +12.4% in 6—24 months (p=0.012). Changes in renal function in long-term period after surgery for pHPT were characterized by decrease of GFR within the limits of chronic kidney disease stages 1—2, also in patients with mild form. Conclusions. Renal concentration capacity impairment is common in mild pHPT and is restorated after surgery for pHPT. The findings of this study add cause for measurement of urine osmolality or osmolality index in all patients with pHPT. Our results confirm the requirement of estimating GFR in pHPT patients not only while active disease, but also in remission after surgery for pHPT. Keywords Mild primary hyperparathyroidism; glomerular filtration rate; chronic kidney disease; renal concentration capacity; osmolality index.