There is some controversy over the need for double data entry in clinical trials. In particular, does the number and types of errors identified with this approach justify the extra effort involved? We report the results of a study carried out to address this question. Our main outcome measure was the frequency and types of errors involved in the entry of data for the CHART (continuous, hyperfractionated, accelerated radiotherapy) trials. Data were reentered for a sample of 44 patients by a data manager other than the one making the initial entry. The second entry was then compared with the first entry. The error rate for the two entries combined was 14 per 10,000 data items (fields) (95% confidence interval 10, 19). The error rate for the initial entry alone was 15 per 10,000 fields (95% confidence interval 9.5, 22), and the vital/important error rate (defined as any error on a principal outcome measure or a major error on any other endpoint or variable) was 2.5 per 10,000 fields (95% confidence interval 0.68, 6.4). On this evidence double data entry is not performed for the CHART trials.