Screening by Northern blot for lck expression in 51 patients with diverse hematologic diseases has shown, for four of them, a 3 to 15-fold increase in the level of lck mRNA when compared with expression in healthy donors. These patients suffered from diverse malignancies: one Burkitt lymphoma, one T-cell lymphoma and two non-Hodgkin B-cell lymphoma. Specific analysis of the different lck transcripts in these patients by polymerase chain reaction and their relative quantitation demonstrate a significant increase of only the type IB and the type IC lck transcripts arising from the proximal promotor. Our study shows: (a) a high lck expression may occur in diverse hematologic diseases, (b) whatever the type of malignancy, this high expression results in a specific increase of the spliced transcripts arising only from the proximal promotor, and (c) in these four patients without any rearrangement or amplification, the high lck expression probably results from the specific activation of the proximal promotor.