Analysis of 5575 settings on a computer‐monitored Theratron‐8060Co unit demonstrates that human error does occur in treating patients with radiation. The errors are due to inaccurate setting of such parameters as field size, gantry angle, collimator rotation, treatment time, etc. The error rate per parameter was found to be about 3%, and more than two‐thirds of the patients monitored with the PDP 11/45 computer had at least one error at some stage during the full course of treatment. Both the dose and the dose distribution may be affected by these errors and have been studied in a few typical cases. The errors in timer setting have the largest effect on the prescribed dose and may change the probability of local control appreciably.