Since 1984 holter monitoring has been used as a routine diagnostic procedure on Clinic for diseases of the heart and rheumatism in Sarajevo. Recently, this procedure is introduced in outpatient praxis and in private outpatient praxis too. However, serious changes on ECG were missed occasionally. 230 patients underwent ECG holter monitoring as outpatient procedure, 24 patients repeated procedure because of suspect incorrect interpretation. 4 of 24 patients had serious changes on ECG monitoring missed on first examination. Patient presented with an chest pain in physical strain: I. recording: in strain, depression of ST in lead III to 1.5 mm with pain, II recording: in strain, ST depression in lead III to 4.4 mm, with pain, in rest, ST depression to 2.5 mm without symptoms of pain. Patient with chest pain in the morning (both in strain and in rest): I. recording: without significant changes (inspite of present pain). II. recording: in strain and in rest occasionally ST elevation in lead II up to 3 mm like Prinzmetal ischemia for 2 to 4.5 min. with chest pain. Female patient with symptoms of weakness and frequent tacchycardia: I. recording in sinus rhythm with episodic absolute tacchyarrhythmia and heart rate up to 135. II. recording showed sinus rhythm, absolute tacchyarrhythmia and heart rate up to 150 and periodical atrial rest with longest RR 2929. Later, pace maker was implanted. Female patients with no symptoms and episodes of arrhythmia: I. recording was with technical difficulties, but seemed to be without changes. II recording: normal finding except in afternoon hours when patients noted ES and holter monitoring recorded spontaneous supraventricular paroxysms up to 22 respectively, with short PQ and heart rate around 190. This implicates need on good technical preparation of patients for ECG-holter-monitoring (for male patients chest depilation) and good placement of electrodes. Doctor should correct and interpret findings not just automatically record dates.