OBJECTIVE Acute heart failure (AHF) may be associated with low-tissue perfusion and/or hypoxaemia leading to increased lactate levels and acid-base perturbations. Few data are available on the clinical significance of elevated lactate levels and primary acid-base disorders in the setting of AHF. RESULTS Arterial blood gas was obtained at admission in 4012 normotensive (systolic blood pressure ≥ 90 mmHg) patients with AHF. The association between lactate levels and acid-base status and in-hospital mortality was determined using multivariable logistic regression. Hyperlactataemia (>2 mmol/L) was present in 38.0% of patients and was strongly associated with markers of sympathetic activation, such as hyperglycaemia. Hyperlactataemia was present in 31.0%, 43.7%, and 42.0% of patients with normal pH, acidosis, and alkalosis, respectively. In-hospital mortality occurred in 16.4% and 11.1% of patients with and without hyperlactataemia [adjusted odds ratio (OR) 1.49; 95% confidence interval (CI) 1.22-1.82, P < 0.0001]. Compared with normal pH, the OR for in-hospital mortality was 2.48 (95% CI 1.95-3.16, P < 0.0001) in patients with acidosis and 1.77 (95% CI 1.32-2.26, P < 0.0001) in patients with alkalosis. The risk for in-hospital mortality was high with acidosis (18.1%) or alkalosis (10.4%) even with normal lactate. The most common primary acid-base disturbances included metabolic acidosis, respiratory acidosis, and metabolic alkalosis with respiratory acidosis having the highest risk for in-hospital mortality. CONCLUSIONS Hyperlactataemia was common in patients without hypotension and was associated with increased risk for in-hospital mortality. Hyperlactataemia is not associated with any specific acid-base disorder. Acute heart failure patients also present with diverse acid-base disorders portending increased in-hospital mortality.