We analyzed the errors occurring in the preparation circuit of cytotoxic mixtures of the Centralized Cytotoxic Preparation Unit during one year. Analysis of their evolution meant the investigation of twenty parameters susceptible to error. Each parameter was considered one error opportunity. Error has been defined either by the lack of data or mistake in the controlled parameter. In 4,734 preparations (94,680 parameters) there were 314 errors. The percentage of error per parameter in the first month of study was 0.74; at sixth was 0.34 and the last month was 0.26. Only in four months the day of maximum number of preparations coincided with the day of maximum number of errors. We conclude that the percentage of errors in the preparation process is low with a tendency to decrease and that the number of daily preparations is not the single factor that influences the production of errors.