INTRODUCTION: The aim of the present study is to determine the preoperative and operative factors associated with bleeding in percutaneous nephrolithotomy (PNL) operations where tract dilatation was formed by balloon dilators. MATERIAL AND METHODS: A total of 378 patients underwent PNL in our department between 2003 and 2008. After excluding missing data 310 patients were included in the study in whom nephrostomy tract dilatation was performed using balloon dilators. The amount of blood loss was estimated by calculating the change in hemoglobin (Hgb). Preoperative and operative factors were assessed for association with the change in Hgb levels. The preoperative factors were age, sex, hypertension, diabetes mellitus (DM), smoking, ipsilateral pyelonephritis, body mass index, serum creatinine level, stone localization and burden, previous ipsilateral renal stone surgery and extracorporeal shock wave lithotripsy and degree of hydronephrosis. Operative factors were operation time, calyx of puncture and tract number. The possible effect of surgical experience was also taken into consideration. Univariate and stepwise multiple linear regression analysis were performed. RESULTS: The average Hgb drop was 1.9 g/dL (range: 0.1 g/dL-8.8 g/dL). Ancillary procedures were performed due to the residual calculi in 23% of patients. The stone free rates increased from 77% to 94% after the secondary interventions. Prolonged operation time and presence of diabetes mellitus (DM) had a significant association with the decrease in Hgb levels (p < 0.05). However the remaining factors analyzed did not have any association with the change in Hgb values (p > 0.05). Previous ipsilateral open renal stone surgery was not associated with a change in Hgb levels (p > 0.05). Staghorn stones and operation expertise were the factors related with operation time (p < 0.05). CONCLUSIONS: In patients where tract dilatation is gained by balloon dilators prolonged operation time and DM are the major risk factors related with a higher incidence of blood loss. Surgical expertise is highly related with operation time. It would be wise to operate staghorn stones when a high surgical experience is gained.