OBJECTIVE: The objective of this study was to devaluate the risk of renal cancer in patients with atypical renal cysts and to compare radiological data used to establish the Bosniak classification with clinical or histological data. MATERIAL AND METHOD: We performed a retrospective study on 37 patients managed in our establishment for atypical renal cyst between January 1995 and April 2003. The following criteria were analysed: gender, age, clinical examination and circumstances of discovery imaging findings, Bosniak classification, treatment modalities and follow-up data. These criteria were compared in two populations according to the presence or absence of associated renal cancer. RESULTS: In this series, 6 patients presented a stage II cyst. No cancer was demonstrated in this group of cysts. Ten patients presented a stage IIF cyst and 2 cancers were detected in this group (i.e. 20%). Fourteen patients presented a stage III cyst, with a cancer in 4 cases (30%) and 7 patients presented a stage IV cyst with 6 cancers (86%). CONCLUSION: The Bosniak classification is currently the reference classification fr the diagnosis of cystic diseases of the kidney. Although stages I and II (cysts with minor changes not requiring surveillance) and stages III and IV (suspicious malignant cysts which require surgical exploration) raise few diagnostic problems, stage IIF (indeterminate cyst requiring radiological surveillance) may be the source of diagnostic difficulties with a risk of missing an associated renal cancer.