In Sweden from 1980 to 1995 there was an overall decrease of 56% in mean length of stay (MLOS) for surgical curative breast cancer treatment. The objective of this study was to separate the possible impact of tumour size and lymph node dissemination and changes in surgical procedures. All women diagnosed (n=13 290) with breast cancer between 1982 and 1995 were selected from the Southern Swedish Tumour Register. Data on LOS, diagnoses, and surgical procedures were obtained from the Swedish Hospital Discharge Register. A multi-factorial model was fitted to the data. Discharges where patients were treated with breast conserving surgery had more than two days shorter MLOS (-2.49, 95% CI -1.66) compared with mastectomy. Although TNM data imply a shift from T2 to smaller T1 among operated women the effect on MLOS is negligible when controlled for age, type of operation etc. Changes in clinical practice such as changes in operation technique can explain approximately 13% of the total decrease in MLOS.