Abstract Background Acromegaly is associated with an increased left ventricular (LV) mass, as reported in echo-based and, more recently, in a few cardiac magnetic resonance imaging (MRI) studies. One possible explanation for this increased LV mass could be water retention and subsequent myocardial edema. Methods In this prospective cross-sectional study, 26 patients with active acromegaly before and after treatment and 31 controls of comparable age and sex were investigated using cardiac MRI. Cardiac morphology, function, and myocardial tissue characteristics were evaluated. Myocardial T2 relaxation time was used as the main outcome measure of myocardial edema. The study was registered with clinicaltrials.gov (NCT02948322). Results Patients compared to controls had greater LV mass indexes (58.1 [54.7-68.6] vs 46.0 [41.3-49.8] g/m2; P < .001) and end-diastolic volume (EDV) indexes (97.3 [88-101.2] vs 81.6 [78.1-96.2] mL/m2; P = .0069) and had comparable global contractile function. T2 values were not different between patients and controls. Both intracellular (43.83 [41.0-50.0] vs 34.32 [28.9-38.7] g/m2; P < .001) and extracellular (15.06 [13.5-17.1] vs 11.6 [10.8-12.7] g/m2; P < .001) LV mass indexes were higher in patients compared to controls. Log growth hormone correlated with myocardial mass (r = 0.75; P < .001). Sex, systolic blood pressure (BP), and the presence of acromegaly were predictors of the LV mass index. The extracellular LV mass index was associated with sex and the presence of acromegaly, whereas the intracellular LV mass index was associated with sex, systolic BP, and high-density lipoprotein (HDL) cholesterol. Acromegaly treatment reduced EDV and total and intracellular LV mass indexes without significantly affecting extracellular mass. Conclusion Acromegaly results in a disease-specific form of LV hypertrophic remodeling, characterized by an increase in both intra- and extracellular mass. The LV mass index and intracellular mass were decreased by treatment.