Sulofenur is a novel diarylsulfonylurea with proven anti-tumor activity in murine tumor models. In this phase II study in patients with advanced gastric or gastroesophageal adenocarcinoma, 17 patients were treated with sulofenur orally at 700 mg/m2 for 14 days every 3 weeks. No tumor responses were seen. The main toxicities were anemia, methemoglobinemia and abnormalities in liver function tests. These toxicities precluded dose escalation. However, plasma levels of sulofenur, and its hydroxy and keto metabolites were probably insufficient to exert anti-tumor effect in comparison with data from murine studies. Further structure-activity studies are warranted.