The International Prognostic Index identifies four risk groups with different survival rates in aggressive non-Hodgkin's lymphoma. We have studied whether a slight modification of this index has prognostic significance in high grade gastric B-cell MALT lymphoma. In 53 patients with high grade gastric B-cell MALT lymphoma the following survival factors were investigated: age over or under 60 years, sex, B symptoms, more than one extranodal site of involvement other than the stomach, serum LDH levels, performance status, stage I/IIE1/IIE2 v.s. stage III/IV, treatment with surgery, chemotherapy or both modalities together and the four risk groups as defined by the Modified International Prognostic Index (MIPI). A multivariate Cox's test was used to evaluate the independent prognostic significance on survival of all the above variables. Advanced stage (III/IV) and involvement of more than one extranodal site not including stomach were the only variables influencing survival. The MIPI was not sufficient to separate groups with significant differences in survival or to stratify prognostic groups. In this series, the MIPI did not show prognostic significance in high grade gastric B-cell MALT lymphoma.