['To determine breast cancer case counts, on a given data set, using both Surveillance, Epidemiology, and End Results (SEER) program and International Agency for Research on Cancer (IARC) multiple primary coding rules and to describe differences, if any, by age at diagnosis, histology, stage at diagnosis, laterality, and grade.', 'SEER and IARC multiple primary coding rules were applied to a dataset provided by the North American Association of Central Cancer Registries. Only registries whose data met high quality data standards for the time period studied (1994-1998) and whose permission was obtained were included. Percentage differences were calculated using IARC counts as the base.', 'Using IARC multiple primary rules resulted in 2.4% fewer cases. Among females, the highest percent changes by category were: age group - 80-84 year-olds (3.4%); histology - inflammatory breast cancer (4.6%); stage - distant (3.1%); grade - well differentiated (3.0%). Among males, the highest percent changes by category were: age group - 80-84 year-olds (1.7%); histology - for intraductal and lobular breast carcinoma in combination (4.8%); stage - distant (3.0%); grade - well differentiated (1.8%). Overall differences were generally unaffected when examined by laterality.'] Does variation in breast cancer count using SEER and IARC multiple primary coding rules?