Cardiovascular diseases are among the leading causes of mortality in patients with various malignancies. While cardiac dysfunction linked to antitumor therapy has received considerable attention in the literature, arrhythmia remains an important complication—ranging from benign to life-threatening. This prospective study followed 127 non-Hodgkin lymphoma patients, assessed by 24h Holter ECG before and six months after antitumoral treatment. The most prevalent arrhythmic events detected were supraventricular tachycardia, ventricular and supraventricular extrasystoles. The PQ interval did not change overall during the study, but it was significantly prolonged in patients receiving high-dose cyclophosphamide (p < 0.001), with no effect observed from doxorubicin. The QTc interval increased significantly from 403.2 ms to 432.8 ms (p < 0.001), showing a moderate correlation with doxorubicin dose and a stronger association with treatment regimens that included cyclophosphamide (p < 0.001). Approximately 10% of patients developed a QTc ≥ 480 ms (p = 0.0026), and 17.4% had QTc values between 450–480 ms (for men) and 460–480 ms (for women) (p = 0.0001). These changes were mainly linked to high-dose cyclophosphamide, with no significant correlation observed for doxorubicin. During antitumor treatment, arrhythmic events increased, correlating with high doses of doxorubicin and cyclophosphamide. High-dose cyclophosphamide was linked to prolonged PQ and QTc intervals and more frequent QTc ≥ 480 ms.