Spontaneous remission of simple renal cyst or complex renal cyst Bosniak III or IV class is a rare phenomenon. Our report describes a spontaneous partial regression of complex renal cyst Bosniak IV during the observation of complex cystic renal masses. A 66 year old male high risk patient (ASA score 3-4) refused all surgical intervention and was suggested for active surveillance strategy. We observed spontaneous partial regression of the complex cyst in this patient. To our knowledge this is the first report of spontaneous partial regression of complex renal cyst Bosniak IV class (Fig. 3, Ref. 5). Full Text (Free, PDF) www.bmj.sk.