A long-term multi-center quality control study of CA15-3 determinations based on measurements of liquid BIOREF CA15-3 control sera was conducted in 17 participating laboratories. Seven different CA15-3 assays were applied using the appropriate automatic immunoanalyzers. CA15-3 means were determined for BIOREF low, medium and high level control sera. Values were 19.3 ± 2.7 kU/l, 75.2 ± 11.4 kU/l and 162.9 ± 37.1 kU/l, respectively. Inter-assay imprecisions were calculated for each of the controls for each laboratory and for each of the methods, with coefficients of variation (CV) ranging from 2.9-15.5%. As a means of evaluation of assay linearity concentration ratios (high/medium, medium/low, high/low) were calculated and found to be in good agreement with refer- ence values throughout the study. Individual long-term time courses of CA15-3 control measurements provided evidence for variability of test results due to changes in assay calibration. Comparisons with CV data obtained with BIOREF controls 17 years ago demonstrate significant improvements of CA15-3 assay precision in recent years. In conclusion, test-independent reference material can be used for CA15-3 quality control and in particular enables applicants to check for long-term stability of CA15-3 assay performance.