Protein tyrosine kinase (PTK) activity was assayed in cytosolic extracts from normal breast tissue, benign tumors, and 84 T1-T2, N0-N1 M0, breast carcinomas. Normal breast tissue extracts yielded an average value of 1.9 +/- 1.1 pmol 32P incorporated/min/mg protein, whereas a mean of 12.5 +/- 6.1 was obtained for cancer samples. With a median follow-up of 34 months, in the series of 40 patients classified N-, PTK positive patients presented a significantly smaller 3-year disease free survival than the PTK negative ones. Multivariate analysis shows that PTK activity emerges as a potential prognostic factor in breast cancer (p = 0.02). These preliminary results will be updated on a bigger cohort of patients.