<p>In this case report, we describe an atypical clinical course in a dog with preexisting myxomatous mitral valve disease (MMVD) subsequently complicated by the development of hyperadrenocorticism. Typically, hyperadrenocorticism can exacerbate cardiac conditions; however, this dog demonstrated paradoxical improvement. A retrospective analysis of the clinical, radiographic, echocardiographic, and hormonal data was conducted to investigate the marked reduction in cardiac size in a Maltese dog with MMVD following the diagnosis of hyperadrenocorticism. A 10-year-old spayed female Maltese dog presented with dyspnea. The initial clinical presentation, diagnosis of MMVD, subsequent identification of hyperadrenocorticism, and observed changes in cardiac size were comprehensively documented. At initial presentation, the dog exhibited severe cardiomegaly and left-sided heart failure due to mitral valve prolapse associated with MMVD and resulting severe mitral regurgitation. Nineteen months later, hyperadrenocorticism was diagnosed. Subsequent echocardiography revealed ventricular hypertrophy and diastolic dysfunction, which may have contributed to changes in mitral valve geometry and reduced mitral regurgitation. Echocardiography revealed a significant reduction in cardiomegaly and an improvement in hemodynamic parameters. Diuretic therapy was gradually discontinued, and the dog was successfully managed with pimobendan and trilostane. This case report highlights the complex interplay between hyperadrenocorticism and MMVD and demonstrates an atypical clinical outcome characterized by cardiac improvement.</p>