Detailed root cause analysis to determine causes of pulmonary cytology errors has not been used to design specific practice changes. We performed root cause analysis of all false-negative bronchial brushing and washing specimen errors (n = 32) detected by the cytologic-histologic correlation process in 2002. Medical records and all slides were reviewed. Based on the correlation process, 10 errors were interpretive, 16 sampling, and 6 combined interpretive/sampling. Root cause analysis showed that the lesion was not accessible in 8 cases and tumor was readily identified on the slides in only 1 case. In 11 cases, the malignant cells were few and not recognized, and in 13 cases, obscuring artifacts (eg, cellular crushing and air drying) limited interpretation. Sampling issues had a major role in the misdiagnosis in 31 cases (97%), and recommendations for error reduction include immediate interpretation and the use of transmucosal fine-needle aspiration.