CDC25 phosphatases activate cyclin-dependent kinases by removing inhibitory phosphate groups on the molecules and positively regulate the cell cycle progression. The expression of CDC25A, B and C was examined in gastric carcinoma cell lines and gastric carcinoma tissues by northern blotting and immunohistochemistry. The gastric carcinoma cell lines expressed CDC25A, B and C mRNA at various levels. The expression levels of CDC25B were generally higher than those of CDC25A and C. Of the 40 gastric carcinomas, 70% of the tumors expressed CDC25B mRNA at higher levels than the corresponding normal mucosas, while 38% overexpressed CDC25A mRNA. The CDC25C expression was at very low or undetectable levels. No obvious correlation was detected between the expression of CDC25B and p53 gene mutations. Immunohistochemically, CDC25-positive tumor cells were detected in 43 (78%) of 55 gastric carcinoma cases, of which 27 (49%) were strongly positive. Strong expression of CDC25B protein was associated with advanced stage and deep invasion. Furthermore, the incidence of strong expression was significantly higher in carcinomas with nodal metastasis than in those without metastasis. These findings suggest that overexpression of CDC25B may favor development and progression and may be an indicator of malignant behavior of gastric carcinomas.