A 45-year-old healthy woman presented to our hospital because of an abnormality on electrocardiography (ECG).  She did not complain of chest pain and she had no history of hypertension, diabetes mellitus, dyslipidaemia, chronic medications, or trauma.  Her blood pressure was 111/79 mmHg.  A 12-lead ECG (*[Figure 1](#ytaa018-F1){ref-type="fig"}*) revealed a heart rate of 70 b.p.m.  and an ST-segment elevation (1 mm) in leads II, III, aVF, and V1--V6.  The morphology of the ST-segment elevation was horizontal.  The inverted P waves in leads II, III, aVF, and V1--V6, and the upright P waves in leads aVR and aVL, suggested an ectopic atrial rhythm.  Echocardiography revealed normal left and right ventricular size, wall thickness, and wall motion.  The left and right atrial size were normal.  No evidence of valvular lesions was found.  The electrolyte levels were normal.  When the P wave is negative and the PR interval is short, positive atrial repolarization waves (Ta) extending into the ST segment can cause a so-called false-positive ST-segment elevation.  As she had no symptoms, we did not prescribe any medications, including antiarrhythmic drugs.  The second ECG (*[Figure 2](#ytaa018-F2){ref-type="fig"}*), obtained 2 months after the initial presentation, revealed resumption of sinus rhythm (a heart rate of 77 b.p.m.)  and complete resolution of the ST-segment elevation. 

![ The initial 12-lead electrocardiogram shows a heart rate of 70 b.p.m.  and ST-segment elevation in leads II, III, aVF, and V1--V6.  The inverted P waves in leads II, III, aVF, and V1--V6, and the upright P waves in leads aVR and aVL, suggest an ectopic atrial rhythm.](ytaa018f1){#ytaa018-F1}

![ The second electrocardiogram obtained 2 months after the initial presentation shows resumption of sinus rhythm (a heart rate of 77 b.p.m.)  and complete resolution of the ST-segment elevation.](ytaa018f2){#ytaa018-F2}

The atrial T wave (Ta) is usually directed opposite to the main P-wave axis.  The Ta wave is frequently recognized in the presence of an atrioventricular block.  The duration of the Ta wave ranged from 230 to 384 ms.[@ytaa018-B1] Atrial repolarization extending into the ST segment can cause a false-positive ST-segment depression,[@ytaa018-B2]^,^[@ytaa018-B3] particularly in the presence of a short PR interval.  Knowledge that a combination of negative P waves and significant positive Ta waves can mimic ST-segment elevation conditions such as myocardial infarction is important. 

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**Conflict of interest:** none declared.