Recent in vitro studies have shown that interleukin 4 (IL-4) induces and gamma interferon (IFN-) inhibits collagen production. To define the TH1(IFN-) and TH2(IL-4) cytokine profiles in systemic sclerosis (Sscl), a disease characterized by widespread fibrosis, we investigated IL-4 and IFN- transcripts in peripheral blood mononuclear cells and plasma protein levels in 13 patients with Sscl. Two previously identified IL-4 transcripts, a full-length transcript and an alternatively spliced (truncated) transcript (designated IL-42), were identified in patients and normal controls. Significantly increased levels of total IL-4 transcripts (full-length plus IL-42 transcripts) were found in patients with Sscl in comparison to those found in healthy controls (P = 0.003), and this increase was primarily due to an increase in the level of the alternatively spliced IL-42 form. The IL-42/full-length-IL-4 transcript ratio was significantly increased in Sscl patients (P < 0.0001, versus healthy controls). Sequencing analysis revealed that the frequency of IL-4 clones carrying the IL-42 transcript was also substantially increased in patients with Sscl. Plasma IL-4 protein levels were increased in Sscl patients compared to those in healthy controls (P = 0.001) and correlated with total IL-4 transcript levels. The up-regulation of the fibrogenic IL-4 (a TH2 cytokine) in Sscl suggests a pathogenic role for IL-4 in this disease.