The inter-physician and inter-patient variability in planning target volume delineation for the radiotherapy of breast cancer after conservative surgery is presented. Eleven experienced radiation oncologists determined the planning target volume (PTV) for four breast cancer patients. Delineation was based on CT slices taken at intervals of 15 mm. The variability in target volume delineation was determined by measuring the volumes in units of cc and the position of the drawn PTVs. Statistical analysis was based on X/R-charts and on Pareto chart and analysis. The maximum range in PTV for one patient was from 670 to 1,200 cc. The observations of three physicians were in excess of the warning limit altogether 18 times. The methods used in this study clearly reveal inter-physician variability in PTV delineation and widest variations found are not acceptable. Training targeted to some physicians and more detailed and unambiguous protocols for PTV delineation are needed.