In order to better understand the potential malignancy of renal neoplasms arising in patients with acquired renal cystic disease and to try and establish differences from other renal tumours we analysed DNA ploidy as well as the level of S-phase fraction in 11 neoplasms associated with acquired cystic disease by means of flow cytometry. The results were correlated with known prognostic factors such as nuclear grade, size and stage, as well as the clinical behaviour of the tumours. We found a close relationship between DNA aneuploidy and high S-phase fraction and a poor clinical outcome. We also found some differences in the DNA ploidy profile of these tumours when compared with those reported in other renal neoplasms.