Data are non-existent regarding coincidental alterations in the expression of p53 and its downstream target genes MDM2 and p21 Waf1/Cip1 in gastric carcinogenesis. An immunohistochemical study was therefore performed to examine the interrelationships of p53, MDM2, and p21 Waf1/Cip1 expression in a series of Caucasian early gastric carcinomas and precursor lesions. In normal gastric mucosa, chronic gastritis, and intestinal metaplasia, the surface cells expressed p21 Waf1/Cip1 in the absence of detectable nuclear p53 and MDM2 protein. Nuclear p53 protein accumulation was found in 60 per cent of the carcinomas, with significant differences in staining characteristics between the Lauren types in the absence of detectable MDM2 protein (p<0β’005). Nearly 80 per cent of the carcinomas expressed p21 Waf1/Cip1 , irrespective of Lauren type. Stratification of the carcinomas according to histological grade and growth pattern did not result in significant differences in p53 and p21 Waf1/Cip1 expression. Finally, no significant correlation was found between overall p53 and p21 Waf1/Cip1 expression in early gastric carcinomas. It is concluded that p21 Waf1/Cip1 expression in the non-neoplastic mucosa most likely relates to cell senescence and/or terminal differentiation, perhaps even in a p53-independent manner. In view of p53/MDM2 homeostasis, the differences in p53 staining characteristics between intestinal and diffuse-type carcinomas probably result, at least in part, from a difference in the prevalence of p53 gene mutations. Moreover, p53-independent induction of p21 Waf1/Cip1 expression apparently occurs in a considerable proportion of early carcinomas. Finally, in contrast to other carcinomas, p21 Waf1/Cip1 expression is not significantly correlated with histological grade in gastric carcinomas, suggesting possible defects downstream of p21 Waf1/Cip1 as an underlying cause for this apparent discrepancy.