Research demonstrated nurses' education needs to be able to document nursing diagnoses, interventions and patient outcomes in the EHR. The aim of this study is to investigate the effect of Guided Clinical Reasoning, a learning method to foster nurses' abilities in using standardized language. In a cluster randomized experimental study, nurses from 3 wards received Guided Clinical Reasoning (GCR), a learning method to foster nurses in stating nursing diagnoses, related interventions and outcomes. Three wards, receiving Classic Case Discussions, functioned as control group. The learning effect was measured by assessing the quality of 225 nursing documentations by applying 18 Likert-type items with a 0-4 scale of the measurement instrument "Quality of Nursing Diagnoses, Interventions and Outcomes" (Q-DIO). T-tests were applied to analyze pre-post intervention scores. GCR led to significantly higher quality of nursing diagnosis documentation; to etiology-specific nursing interventions and to enhanced nursing-sensitive patient outcomes. Before GCR, the pre-intervention mean in quality of nursing documentation was = 2.69 (post-intervention = 3.70; p < .0001). Similar results were found for nursing interventions and outcomes. In the control group, the quality remained unchanged. GCR supported nurses' abilities to state accurate nursing diagnoses, to select effective nursing interventions and to reach enhanced patient outcomes. Nursing diagnoses (NANDA-I) with related interventions and patient outcomes provide a knowledgebase for nurses to use standardized language in the EHR.