The knowledge about diagnostic reasoning of baccalaureate nurses and undergraduate students is important to the development of educational strategies. This study's objectives included to culturally adapt the Diagnostic Thinking Inventory (DTI) for the Brazilian culture, analyze its psychometric properties, and describe the diagnostic rationale nurses and nursing students with selected variables. The DTI is a Canadian inventory based on the theory of hypothesis generation, created to measure the diagnostic ability. The inventory has two subsections (flexibility in thinking and evidence structure knowledge in memory). The DTI's translation process resulted in a Brazilian version applied to a sample of 83 nurses (28,9%); average age of 29,7 ± 6,6 years, and 205 students (71,1%); average age of 24,7 ± 5,61years. The results of the confirmatory factor analysis concerning a moderate fit for the DTI model ( 2 = 1369; GFI= 0,793; AGFI= 0,771; RMSEA= 0,053; NFI= 0,458; NNFI= 0,635; CFI= 0,654 e SRMR= 0,068) and the internal consistence (Cronbach's alpha) showed a good internal consistency to total score (0,801), flexibility (0,635) and evidence (0,742). Person's coefficient of correlation showed that the DTI has good reproducibility over time (0.806; p=0,001). No have difference between nurses' flexibility scores (4,1±0,48; IC 95% 3,98-4,18) and students' scores (4,2±0,51; IC 95% 4,1-4,3) (p=0215). No have too difference between nurses' evidence structure scores (4,3±0,59; IC 95%, 4,1-4,4) and students' scores (4,3±0,53 IC 95% 4,2-4,4) (p=0,742).The variables applied together with the DTI presented significant differences: nursing diagnosis in graduate course (flexibility p=0,001; evidence structure p=0,009); clinical reasoning in graduate course (flexibility p=0,031; evidence structure p>0,001); nursing diagnosis with read and research (evidence structure p=0,001); nursing diagnosis with clinical practice (evidence structure p<0,001); self-evaluation of clinical reasoning ability (flexibility p= 0,003; evidence structure p< 0,001) and for only nurses, the diary clinical practice with use of nursing diagnosis (evidence structure p<0,001).The analysis' results lead to the conclusion that to use and to teach about nursing diagnosis is very important to diagnostic reasoning in nursing, although other studies are needed to confirm or adjust the Brazilian version of the DTI.