OBJECTIVE: Our purpose was to determine whether protein tyrosine phosphatase 1B is overexpressed in ovarian cancers, possibly altering the balance of intracellular tyrosine phosphorylation. STUDY DESIGN: The expression of protein tyrosine phosphatase 1B was assayed in frozen sections from 54 human ovarian carcinomas and seven normal ovaries by immunochemical staining with monoclonal antibody AE4-2J, which is specific for protein tyrosine phosphatase 1B. The expression of protein tyrosine phosphatase 1B-specific messenger ribonucleic acid in tumors was determined by Northern analysis. The results were analyzed statistically by means of Fisher's exact test. RESULTS: Minimal staining was observed in normal ovarian epithelium. In contrast, 43 of 54 (79.6%) tumors displayed increased protein tyrosine phosphatase 1B expression, which is statistically associated with malignancy. Overexpression was associated with the expression of the p185c-erbB-2, p170EGFR, and p165mCSFR growth factor receptor protein tyrosine kinases. Protein tyrosine phosphatase 1B messenger ribonucleic acid expression was inconsistently increased in tumor cells. CONCLUSION: Increased expression of protein tyrosine phosphatase 1B in ovarian cancers that also express protein tyrosine kinases suggests that protein tyrosine phosphatase 1B may play a role in the growth regulation of ovarian cancers.