The treatment of diastolic dysfunction in a subject and, in particular, compositions for use and methods in treating diastolic dysfunction are disclosed. In one aspect, the disclosure provides a composition for use comprising a therapeutically effective amount of a substance that increases the level and/or activity of a crystalline protein in cardiomyocytes of the subject. In particular, the composition for use according to the disclosure comprises a therapeutically effective amount of the alpha B crystalline protein. It was found that the composition for use is capable of treating diastolic dysfunction and diastolic heart disease in subjects with failing hearts that have a higher cardiomyocyte stiffness than controls. Particularly, addition of alpha B crystalline reduced the higher stiffness of failing cardiomyocytes to the level observed in control cardiomyocytes.