Few studies have measured the effect of pre-sign out double viewing of cytology cases as a means to decrease error. Three Agency for Healthcare Research and Quality-funded project sites performed pre-sign out double viewing of 431 pulmonary cytology cases. Two-step or more differences in diagnosis were arbitrated as interpretive errors, and the effect of double viewing was measured by comparing the frequency of cytologic-histologic correlation-detected errors in the previous 2 years with the double-viewing period. The number of interpretive errors detected by double viewing for the 3 institutions was 2.7%, 0% and 1.9%, respectively. Double viewing did not lower the frequency of cytologic-histologic correlation false-negative errors. We conclude that double viewing detects errors in up to 1 of every 37 cases and that biases in the double-viewing process limit error detection.