The helicobacter-associated transition from chronic gastritis to MALToma (lymphoma of mucosa-associated lymphoid tissue) may require genetic change in the host. We have studied gastrectomy specimens from twelve cases of primary B-cell gastric lymphoma showing evidence of chronic gastritis and low-grade or high-grade MALToma to look for allele imbalance at microsatellites for six tumour-suppressor genes. We detected allelic imbalance at two of these loci (DCC in three, APC in two). In two DCC cases allele imbalance was seen in the transition from chronic gastritis to low-grade MALToma and in the third between low-grade and high-grade. Allele imbalance between chronic gastritis and low-grade MALToma is not necessarily causal in the transition. Rather, genetic change has occurred in the process of transformation.