: We aimed to explore left atrial (LA) remodeling in the patients with solid cancer before initiation of chemo- or radiotherapy. This retrospective investigation included 92 chemo- and radiotherapy-naive cancer patients and 40 age- and gender-matched controls with a similar cardiovascular risk profile as the cancer group. All participants underwent comprehensive echocardiographic examination before the start of chemo- or radiotherapy. LA phasic function was evaluated in volumetric and strain method. Indexed minimal and pre-A LA volumes were significantly higher in the cancer patients. Total and passive LA emptying fraction (EF) were significantly lower, whereas active LAEF was significantly higher in the cancer patients. LA total longitudinal strain was significantly lower in the cancer patients. Strain rate analysis of LA phasic function showed that LA function during systole and early diastole was reduced in the cancer group, while it was increased during late diastole. These findings indicated that LA reservoir and conduit functions, assessed with LA volumetric and strain analysis, were deteriorated in the cancer group. On the other hand, LA booster pump function was elevated in the cancer group in comparison with the controls. In the whole population, cancer was associated with reduced LA total longitudinal strain independently of age, gender, BMI, LV hypertrophy, E/e' ratio, diabetes, and hypertension. LA phasic function was impaired in the chemo- and radiotherapy-naive cancer patients in comparison with the control group. Cancer, LV hypertrophy, and hypertension were associated with reduced LA longitudinal strain independently of other important clinical parameters.