The present invention generally provides a therapy for effectively treating and/or preventing diseases associated with cells expressing CLDN18.2, in particular cancer diseases such as gastroesophageal cancer. Data are presented demonstrating that administration of an anti-CLDN18.2 antibody to human patients with gastroesophageal cancer is safe and well-tolerated up to a dose of at least 1000 mg/m2. Furthermore, data are presented demonstrating that the antibody is fully functional in these patients to execute anti-tumor cell effects and evidence for antitumoral activity was obtained.