An experiment was conducted to determine whether, using microcomputer-based data entry, double data entry (DE) significantly lowers data entry keying error rates when compared to single entry (SE). Clinical centers of the Cardiac Arrhythmia Suppression Trial (CAST) participated in a randomized crossover design experiment comparing SE and DE. A total of 42,278 data items (fields) were checked for consistency between the paper data form and the computer database. The overall error rate was 19 per 10,000 fields. Error rates were 22 and 15 per 10,000 fields for SE and DE, respectively; P = .09 by Poisson regression. DE took 37% longer than SE, costing each clinic approximately an extra 90 min per month.