Since different histological types (HT) of gastric cancer (GC) may differ in their aetiology, time trend analysis by HT may afford an insight into aetiology. From the Gastric Cancer Registry of Japan, 161 067 cases diagnosed were retrieved between 1975 and 1989 to calculate the annual relative frequencies, stratified by age group and sex, of HT according to the Lauren and the Japanese Research Society for Gastric Cancer (JRSGC) classifications. Age- and sex-specific incidence rates by HT were estimated by multiplying the corresponding national cancer incidence rates of GC by the relative frequencies. Logistic regression models stratified by sex and age group were fitted to determine the time trends of HT. Using the Lauren classification, a decreasing trend of the intestinal type and a stable trend of the diffuse type were found. By the JRSGC classification, significant decreasing trends for most age groups were found for papillary and mucinous adenocarcinomas. Tubular adenocarcinomas (well differentiated type) showed a decreasing trend only in younger age groups. Tubular (moderately differentiated type), poorly differentiated adenocarcinomas, and signet ring cell carcinoma were statistically stable during the period. Considering changes in lifestyles of the Japanese, the result suggests that there are three aetiological types of GC. © 2001 Cancer Research Campaign http://www.bjcancer.com