To assure the quality of data collected from breast cancer screening, a data validation process is essential. The validation will take place in two stages. The syntactic correctness of records and the existence of unique identification numbers of clients and examinations will be verified during the primary validation. Duplicity records will be eliminated too. Only records meeting given requirements will be stored in the central database. Then secondary validation – checking of logical connections between records – will take place over the central database. Tight cooperation between the statistic centre and project centres is necessary to guarantee required effectiveness of the process. All centres will be informed about mistakes and discrepancies bymeans of so called validation reports.Records will bemanually corrected onthe basis of responses from centres. The validation system has been created and the testing is running over the data from more centres these days.

        Key words: mammographic screening – data validation – duplicity