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OBJECTIVES: To assess the impact of using data from the National Death Index (NDI) of Spain to estimate breast cancer survival rates among residents of Girona and Zaragoza diagnosed in 1995-1999. METHODS: This was an observational, longitudinal epidemiologic study, using two population- based cancer registries. Data collected were of female residents of Girona or Zaragoza who had been diagnosed with breast cancer in 1995-1999. Observed and relative 5-year survival rates were estimated, first using the information available from the Girona and Zaragoza cancer registries, and then with the inclusion of NDI data. The 5-year relative survival rate and corresponding 95% Confidence Intervals were estimated using the Hakulinen method. The Kaplan-Maier method and Log Rank test were used to compare survival curves. RESULTS: No statistically significant difference in survival curves was observed in Girona for the data obtained before and after cross-matching with the NDI. However, there was a significant difference in Zaragoza. A comparison of the relative survival rates of each of the two registries before NDI cross-matching showed differences of 3.9% (5-year) and 16.1% (10-year) between the two, whereas after the cross-match, the difference was only 0.5% (5-year) and 1.2% (10-year). CONCLUSIONS: In Spain it is imperative that there be systematic use of NDI data to supplement cancer registries, so that comparisons of relative survival rates between registries can be improved. [ABSTRACT FROM AUTHOR]