Mycosis fungoides (MF) is a low-grade cutaneous T cell lymphoma of unknown etiology. In this report, the Jak/Stat (Janus kinase/signal transducer and activator of transcription) signaling pathway was investigated in tumor cell lines established from skin biopsy specimens from a patient with MF. Jaks link cytokine receptors to Stats, and abnormal Jak/Stat signaling has been observed in some hemopoietic cancers. In MF tumor cells, a slowly migrating isoform of Stat3, Stat3(sm), was found to be constitutively activated, i.e., (i) Stat3(sm) was constitutively phosphorylated on tyrosine residues, and tyrosine phosphorylation was not enhanced by growth factor stimulation; (ii) band shift assays and immunoprecipitations of DNA/Stat complexes showed constitutive DNA-binding properties of Stat3(sm); and (iii) Stat3(sm) was constitutively associated with Jak3. The abnormal activation of Stat3(sm) was highly specific. Thus, neither the fast migrating isoform of Stat3 (Stat3(fm)) nor other Stats (Stat1, Stat2, and Stat4 through Stat6) were constitutively activated. The Jak kinase inhibitor, tyrphostin AG490, blocked the constitutive activation of Stat3(sm) and inhibited spontaneous as well as interleukin 2-induced growth of MF tumor cells. In conclusion, we have provided evidence for an abnormal Jak/Stat signaling and growth regulation in tumor cells obtained from affected skin of an MF patient.