Simple renal cysts are common in older patients and they are usually left untreated, yet only a few studies have been done on the natural history of simple renal cysts. We investigated the sequential changes in their size and number in 103 patients.

From January 1993 to August 2003, we followed 103 patients who were suffering with simple renal cysts. Their mean age was 55.8±18.72 years old (M:F=58:51). Using Abdominal USG and CT, the annual changes in renal cysts were analyzed through the follow-up period, and the mean follow-up period was 52.3±17.58 months. The values of the parameters were compared with using a 2-tailed Student's t-test, multivariate logistic regression analysis and the chi-square test.

At the time of diagnosis, the mean number and size of simple renal cyst were 1.5±1.31 cysts per each kidney and 25.3±12.42mm, respectively. On the consecutive imaging study, the size of the renal cysts was increased 4.4±1.54mm per year. The average size increase was significantly greater in the individuals younger than 50 versus the patients 50 years or older (5.7±2.14mm/year vs 3.7±1.45mm/year, respectively). The multiple cysts showed a significantly larger size increase than did the single cysts (8.1±3.27mm/year vs 3.6±1.64mm/year, respectively). The size increase of the bilateral renal cysts was significantly larger than that of the unilateral renal cysts (5.6±1.48mm/year vs 4.1±1.02mm/year, respectively). No significant difference was observed for the average size increase with regard to gender or the initial cyst size. Only the number of cysts was a significant predictor of aggressiveness on the multivariate logistic regression analysis.

The simple renal cysts progress in size and they appear to grow with aging. They seem to grow more rapidly in patients with a younger age, and for multiple and bilateral renal cysts.