This study documents the prevalence of simple renal cysts at various ages to produce a table of age-specific specificity values. The specificity of diagnosis for inherited renal cystic conditions depends primarily on the prevalence of simple renal cysts in disease-free subjects. To provide specificity data useful for the ultrasound assessment of individuals at genetic risk of having a renal cyst-forming condition, a prospective ultrasound study was performed. The urinary tract was examined in detail in 729 individuals referred for investigation of symptoms unrelated to the urinary tract and who had normal renal function. The prevalence of individuals with at least one renal cyst was 0% in those aged 15 to 29 years, 1.7% in those aged 30 to 49 years, 11.5% in those aged 50 to 70 years, and 22.1% in those aged 70 years and above. The prevalence of bilateral renal cysts (at least one cyst in each kidney) was 1% in those aged 30 to 49 years, 4% in those aged 50 to 70 years, and 9% in those aged 70 years and above. A table is provided that allows specificity estimates for differing diagnostic criteria together with 95% confidence limits for each estimate. In the younger age groups, bilateral renal cysts, particularly bilateral cysts with one or both kidneys having more than one cyst, are relatively uncommon. A reduced threshold for suspecting inherited renal cystic disease in younger individuals at high genetic risk will continue to have high specificity while improving the sensitivity of ultrasound diagnosis.