Seronegative juvenile rheumatoid arthritis and mast cell-associated gastritis. We describe 4 children with seronegative inflammatory arthritis who had persistent, severe nausea and abdominal pain over several months, in spite of vigorous medical therapy, including antacids and histamine H2 receptor antagonists. Endoscopy and biopsy of gastric and duodenal mucosa showed antral gastritis and an increased number of mast cells in 3 of the 4 patients. In the fourth patient, urinary histamine levels were elevated. These findings suggest an association between inflammatory arthritis and localized mast cell disease in some individuals. Further studies are needed to determine whether this association represents an independent syndrome or whether mast cell-related disease is secondary to long-term treatment with nonsteroidal antiinflammatory drugs in children with mild arthritis. 