Objective  This study aimed to identify the types, frequency, and clinical significance of altered interpreta-tion in clinical settings.Methods  Routine outpatient encounters involving Portuguese-speaking Brazilian patients, Japanese doctors, and hospital-provided interpreters were digitally recorded and transcribed. Segments of consecutively interpreted utterances were coded as “altered,” in which the interpreter changed the meaning of the source utterance, or “unaltered (accurate),” and the types and frequency of occurrence were analyzed. Altered interpretations were categorized as clinically negative or positive and classified into four categories: omission, addition, substitution, or voluntary intervention. Incidents resulting from alterations were investigated. Reliability was assessed by examining the correlation between random independently coded samples.Results  In total, 111 encounters were analyzed. The mean segments per encounter was 67.9 (range 14-186), and the mean frequency (standard deviation) of altered interpretations per 100 segments was 46.7 (14.3) for accurate interpretations; 46.1 (17.9) for negative or not significant alterations, including 27.2 (10.3) for omissions, 6.0 (5.0) for additions, 10.4 (6.9) for substitutions, and 2.5 (2.7) for voluntary interventions; 0.0 (0.2) for alterations potentially leading to an incident; and 26.2 (11.9) for positive alterations, including 1.8 (2.6) for positive omissions, 7.7 (4.7) for positive additions, 7.8 (6.3) for positive substitutions, and 8.8 (5.2) for positive voluntary interventions. The frequency of negative alterations was weakly negatively correlated with number of segments per minute (r＝−0.339). Conclusion  Trained healthcare interpreters with fewer clinically significant altered interpretations were effective. Professional medical providers should recognize the importance of positive alterations by healthcare interpreters and collaborate with them to provide safe medical care for foreign patients. To facilitate the provision of appropriate medical care to a growing foreign population with diverse cultural and linguistic needs, undergraduate medical education should teach aspiring medical professionals how to collaborate with healthcare interpreters and foreign patients.