Abstract Aims To investigate the effects of spironolactone on fibrosis and cardiac function in people at increased risk of developing heart failure. Methods and results Randomized, open-label, blinded-endpoint trial comparing spironolactone (50 mg/day) or control for up to 9 months in people with, or at high risk of, coronary disease and raised plasma B-type natriuretic peptides. The primary endpoint was the interaction between baseline serum galectin-3 and changes in serum procollagen type-III N-terminal pro-peptide (PIIINP) in participants assigned to spironolactone or control. Procollagen type-I C-terminal pro-peptide (PICP) and collagen type-1 C-terminal telopeptide (CITP), reflecting synthesis and degradation of type-I collagen, were also measured. In 527 participants (median age 73 years, 26% women), changes in PIIINP were similar for spironolactone and control [mean difference (mdiff): −0.15; 95% confidence interval (CI) −0.44 to 0.15 μg/L; P = 0.32] but those receiving spironolactone had greater reductions in PICP (mdiff: −8.1; 95% CI −11.9 to −4.3 μg/L; P < 0.0001) and PICP/CITP ratio (mdiff: −2.9; 95% CI −4.3 to −1.5; <0.0001). No interactions with serum galectin were observed. Systolic blood pressure (mdiff: −10; 95% CI −13 to −7 mmHg; P < 0.0001), left atrial volume (mdiff: −1; 95% CI −2 to 0 mL/m2; P = 0.010), and NT-proBNP (mdiff: −57; 95% CI −81 to −33 ng/L; P < 0.0001) were reduced in those assigned spironolactone. Conclusions Galectin-3 did not identify greater reductions in serum concentrations of collagen biomarkers in response to spironolactone. However, spironolactone may influence type-I collagen metabolism. Whether spironolactone can delay or prevent progression to symptomatic heart failure should be investigated.