Backgorund & Aims Mushroom poisoning with Amanita phalloides or similar species can lead to liver failure with 10‐30% mortality rates. We aimed at defining the prognostic value of urinary amatoxin quantification in patients with hepatotoxic mushroom poisoning. Methods Data from 32 patients with hepatotoxic mushroom poisoning (Hospital Clínic Barcelona, 2002‐16) in whom urinary amatoxins were determined (ELISA) were retrospectively reviewed. Correlations between urinary amatoxin and collected baseline variables with outcomes including hepatotoxicity (ALT>1000 U/L), severe acute liver injury (ALI, prothrombin <50%), acute liver failure (ALF, ALI and encephalopathy), transplantation/death and hospital length‐of‐stay, were evaluated. Results 12/32 patients developed increased aminotransferase activity. Among the 13/32 amatoxin negative patients, 1 developed ALI and 12/13 no hepatotoxicity. Among the 19/32 amatoxin positive patients, 8/19 (42%) developed hepatotoxicity, including 5 who progressed to severe ALI, of whom 3 developed ALF (2 deaths, 1 transplantation). Urinary amatoxin and prothrombin were independent predictors of hepatotoxicity, ALT peak values (along with age) and hospital length‐of‐stay. In positive amatoxins patients, urinary concentrations > 55 ng/ml (or a baseline prothrombin ≤ 83%), were associated to hepatotoxicity (presented by 8/9 patients with ALT>1000 U/L). Among 5 patients with urinary amatoxin ≥ 70 ng/ml, 4 developed severe ALI. Conclusions In patients with hepatotoxic mushroom poisoning, a negative urinary amatoxin quantification within 72h of intake ruled out the risk of hepatotoxicity in 92% of patients, whereas positive urinary amatoxins were associated with hepatotoxicity and severe ALI. Concentrations >55 ng/ml and ≥ 70 ng/ml were predictive of hepatotoxicity and severe ALI, respectively. See Editorial on Page 1016