Objective: To test the hypothesis that there is an association between simple renal cysts and arterial blood pressure (BP). Patients and methods: The study comprised a review of the case-notes of 115 patients (mean age 67.7 years, range 31-91) in whom renal cyst(s) was an incidental ultrasonographic finding, and 115 age- and sex-matched control patients. The notes were reviewed without knowledge of the origin or treatment. Results: The systolic and diastolic BP were significantly higher in patients with a cyst(s) than in control patients (P = 0.031 and P = 0.0034; mean difference 6.9 mmHg and 5.0 mmHg, respectively). In those aged 30-70 years, the presence of one or more simple renal cysts increased the risk of having a diastolic BP ≤ 95 mmHg from seven in 58 to 17 in 58 patients (P = 0.038). Conclusion: The association between simple renal cysts and higher arterial BP probably arises from underlying renal disease causing both. Occasionally it is possible that renal ischaemia caused by cyst expansion, and therefore increased renin release, generates the elevation in BP. If so, methods to identify such cysts should be developed.