The 17 beta-hydroxysteroid dehydrogenase (17HSD) enzymes are involved in the regulation of the biological activity of sex steroids. We analyzed the expression of 17HSD type 1 and 2 enzymes which catalyze opposite reactions of estrogen metabolism, in normal gastric mucosa and various gastric diseases of 81 tissue specimens. No expression of 17HSD type 1 mRNA was observed in any of the specimens. 17HSD type 2 mRNA expression was observed in the surface and foveolar epithelium of normal gastric mucosa and in the duodenum, while expression was weak or absent in glandular epithelium. Gender did not have an effect on epithelial expression, but 17HSD type 2 mRNA expression decreased with increasing age in both normal and inflamed gastric mucosa (c = 0.6; p = 0.02, Spearman rank correlation). In children, the expression in glandular epithelium was significantly higher than in adults (p = 0.03). Chronic gastritis was associated with decreased expression in surface epithelium (p = 0.023). Regenerating epithelium close to ulcers and active gastritis showed up-regulation (p < 0.03). Type I intestinal metaplasia showed an up-regulation (p = 0.005) comparable to that seen in the duodenum, while type III metaplasia showed decreased expression in comparison with type I metaplasia (p = 0.003). Expression was further down-regulated in cancer (p < 0.003). 17HSD type 2 mRNA expression in gastric and duodenal epithelium suggests that estrogen and androgen inactivation and active progesterone production are physiological features of gastroduodenal mucosa. The higher 17HSD type 2 mRNA expression in normal gastric mucosa of children compared to adults may be associated with increased need for protection against the mucosal load of foreign substances. The down-regulation associated with aging may have relevance in the pathogenesis of gastric cancer.