BACKGROUND: Subclinical diabetic cardiomyopathy (DCM) is frequent in asymptomatic subjects with type 2 diabetes (T2DM). We sought the response of functional and fibrosis markers to therapy in a trial of aldosterone antagonism for treatment of DCM. METHODS: Biochemical, anthropometric, and echocardiographic data were measured in 225 subjects with T2DM. Myocardial function was evaluated with standard echocardiography and myocardial deformation; ischaemia was excluded by exercise echocardiography. Calibrated integrated backscatter and post-contrast T1 mapping from cardiac magnetic resonance imaging were used to assess myocardial structure. Amino-terminal propeptides of pro-collagen type I (PINP) and III (PIIINP), the carboxy-terminal propeptide of pro-collagen type I (PICP) and transforming growth factor beta-1 were measured from peripheral blood or urine to assess myocardial collagen turnover. RESULTS: Diastolic dysfunction was identified in 81 individuals, of whom 49 (25 male, age 60 ± 10 years) were randomized to spironolactone 25 mg/day or placebo therapy for 6 months. Groups were well-matched at baseline. Spironolactone therapy was associated with improvements in diastolic filling profile (Δpeak E wave velocity -4 ± 15 vs. 9 ± 10 ms, P = 0.001; ΔE/A ratio -0.1 ± 0.3 vs. 0.2 ± 0.2, P < 0.001) and cIB values (-21.2 ± 4.5 dB vs. -18.0 ± 5.2 dB, P = 0.026; ΔcIB -5.1 ± 6.8 vs. -1.3 ± 5.2, P = 0.030). ΔcIB was independently associated with spironolactone therapy (β = 0.320, P = 0.026) but not Δblood pressure. With intervention, pro-collagen biomarkers (ΔPINP P = 0.92, ΔPICP P = 0.25, ΔPIIINP P = 0.52, and ΔTGF-β1 P = 0.71) and T1 values (P = 0.54) remained similar between groups. CONCLUSIONS: Spironolactone-induced changes in myocardial structure and diastolic properties in DCM are small, and are unassociated with changes in collagen biomarkers or T1 values. Published on behalf of the European Society of Cardiology. All rights reserved. © The Author 2014. For permissions please email: journals.permissions@oup.com