OBJECTIVE To test the effectiveness of a 12-hr educational course on nursing diagnosis, critical thinking, and clinical reasoning. METHODS This two-group, quasi-experimental study was conducted with 100 practicing nurses, 50 in each group. Attitude was measured by the Positions on Nursing Diagnosis scale. Accuracy of nursing diagnosis was measured with three valid and reliable case studies, and the Lunney scoring method. RESULTS After completion, there was a significant difference in the mean attitude between control and experimental groups, and in the accuracy of nursing diagnosis. CONCLUSIONS Continuing education can improve clinical reasoning, as shown by improved attitudes toward and accuracy of nursing diagnosis. CONCLUSIONS Nurse educators should create educational strategies for practicing nurses to improve clinical reasoning skills.