PTH has been demonstrated to promote renal epithelial cell proliferation and cysts development. The study aimed to evaluate the prevalence of kidney cysts in patients with primary hyperparathyroidism (PHPT). The prevalence of renal cysts diagnosed at abdominal ultrasound examinations in 172 PHPT patients (59.4+/-15.1 yr, mean age+/-SD; female/male 2.8) with preserved renal function was compared with that observed in 210 age- and sex-matched healthy controls. All patients underwent clinical, serum, and urine evaluations, and bone mineralization assessment by dual X-ray absorptiometry. Simple kidney cysts occurred with a higher prevalence in both male and female PHPT patients in comparison with healthy controls (34.9% vs 16.2% p<0.001). Kidney cysts were absent in patients younger than 39 yr, whereas they were present in one third of PHPT patients in their 4th, 5th, and 6th decades, increasing up to 45% after the age of 70. Multiple renal cysts were larger and more frequent than single cysts. PHPT patients with renal cysts were affected by a more active PTH secretion than patients without renal cysts as indicated by significant higher hypercalcemia and lower tubular maximal phosphate (TmP) reabsorption, while renal function, the occurrence of kidney stones, and osteoporosis were similar in both groups. Reduced TmP values were associated with about 3-fold increase in the risk of kidney cysts. In conclusion, simple renal cysts might be considered as a benign kidney complication of PHPT and might be related to the action of the chronic elevated PTH levels on tubular epithelial cells.