Reproducibility of the AVA program (version 3.5) was tested (1) by analyzing the same analog tracing 10 times in 150 patients, and (2) by studying the influence of 12 different sets of prior probabilities (PRP), using the digital data from a total of 2718 patients. The same QRS-T diagnosis plus or minus 8 percentage points was found in patients with high posterior probabilities. Variability for the P wave diagnosis was twice as high as for QRS-T results. Failure to select the appropriate set of PrP markedly influenced reproducibility. This was especially true for right ventricular hypertrophy and pulmonary emphysema. 'Mispriorizing' in a subgroup of 199 patients with well-documented myocardial infarction lowered the total number of correct ECG diagnoses on the average between 4.8 and 35%. These results are inherent to Bayes' theorem where PrP are used to achieve minimum assignment error in populations, but where Prp errors may result in wide classification discrepancies. Without errors in PrP reproducibility by the VA program was found to be very high.