Trimethylamine N-oxide (TMAO), a gut-derived metabolite, has been linked to risk of death in patients with chronic heart failure (HF), but its association with acute HF remains unclear. Suzuki and colleagues measured TMAO levels in plasma samples from 972 patients with acute HF. TMAO improved risk stratification for in-hospital mortality when assessed together with clinical scorings (OR ≥1.13, P ≤0.014). Furthermore, the investigators report that “elevated levels [of TMAO] were associated with poor prognosis at 1 year, and combination of TMAO and [N-terminal pro-B-type natriuretic peptide] provided additional prognostic information”. TMAO was an independent predictor of death, and death and HF, until adjusted for renal confounders.