Summary. Pegylated interferon (IFN), the basis for chronic hepatitis C virus (HCV) treatment, causes depression in 30–40% of patients. The potential for cytokine mRNA patterns from baseline into early treatment to associate with the onset of treatment-induced depression (TID) was examined. Depression was measured by the Beck Depression Inventory at baseline and weeks 2, 4, 8 and 12 of treatment (n = 38). At baseline and weeks 2 and 4, peripheral blood mononuclear cell (PMBC, n = 28), isolated ex vivo, were examined for tumour neurosis factor (TNF)-alpha, interleukin (IL)-1beta and IL-10 mRNA expression. In patients that developed treatment-induced depression, pro-inflammatory TNF-alpha mRNA levels from baseline into week 4 of therapy remained constant (1.1-fold increase); whereas IL-1beta transcripts decreased 3.5 fold. However, corresponding TNF-alpha (3-fold, P < 0.05) and IL-1beta (7.5-fold) transcript expression diminished to a greater extent in the absence of TID. Changes in TNF-alpha mRNA values correlated to the average change in BDI scores over the 12 weeks (r = 0.56, P < 0.05). Concomitantly, anti-inflammatory IL-10 transcript levels decreased in (TID), relative to increased expression in the absence of TID (P < 0.05). The potential influence of IL-10 was observed upon calculation of individual pro- verses anti-inflammatory mRNA ratios. Stable in the presence of depression, TNF-alpha/IL-10 and IL-1beta/IL-10 mRNA ratios declined significantly over time in its absence (P < 0.05). This study suggests that in chronic HCV infection, upon pegylated IFN administration persistent pro-inflammatory cytokine MRNA expression associates with TID. In contrast, therapeutic activation of mechanisms that decrease pro-inflammatory immunity may protect against depression during therapy.