One hundred and three kidneys with calculi in 100 patients, were treated by conservative renal surgery from Jan., 1980 to Dec., 1982. The operative technique consisted of pyelolithotomy, extended pyelolithotomy, dismembered pyelolithotomy, nephrolithotomy (bivalve or anatrophic nephrolithotomy) partial nephrectomy and pyelo-nephrolithotomy. Intraoperative X-ray and coagulum lithotomy were employed when pyelolithotomy was performed. Thirty-five residual calculi in 20 kidneys were observed on postoperative X-ray film. The rate of residual calculi was 19.4%. Factors causing residual calculi, were analysed on these 103 kidneys. The factors were as follows; the shape of calculi: staghorn calculus with multiple small calculi, the shape of the renal collecting system: narrow pelvis with narrow caliceal neck and dilatated calices, and the operative technique: nephrolithotomy. These results suggested that it would be necessary to minimize residual calculi when performing nephrolithotomy.