The proinflammatory cytokine TNFα (tumor necrosis factor α) plays an important role in the remodelling of the heart that occurs following myocardial infarction. In addition to exerting direct effects on cardiac cell function, TNFα contributes to adverse myocardial remodelling by increasing production of other proinflammatory cytokines, such as interleukin (IL)-1 and IL-6. Thiazolidinediones (TZDs) are insulin-sensitizing agents primarily used for treating Type 2 diabetes. However, in addition to improving insulin resistance, TZDs also exert beneficial pleiotropic anti-inflammatory effects and can reduce adverse myocardial remodelling. The aim of the present study was to determine whether TNFα could stimulate expression of other proinflammatory cytokines in cultured human cardiac fibroblasts, to examine the underlying intracellular mechanisms, and to investigate the modulatory effects of TZDs. Human cardiac fibroblasts were cultured from biopsies of right atrial appendage. Cytokine mRNA expression and secretion was measured using quantitative real-time RT-PCR and ELISA. Activation of signalling pathways was determined by immunoblotting with phospho-specific antibodies. TNFα (0.1-10 ng/ml) stimulated IL-6, IL-1α and IL-1β mRNA expression in human cardiac fibroblasts in a concentration-dependent manner. The use of pharmacological signalling pathway inhibitors (PD98059, SB203580, LY294002 and IMD-0354) and receptor neutralising antibodies established that both TNFα-induced IL-6 and IL-1β expression were mediated via the TNF-RI receptor and p38 MAP kinase, PI3K/Akt and NF-κB pathways. In contrast, TNFα-induced IL-1α expression was mediated via both TNFRI and TNFRII subtypes and stimulated via the p38 MAP kinase and PI3K/Akt pathways, but negatively regulated by the NF-κB pathway. Despite TNFα increasing mRNA levels of all three proinflammatory cytokines, only IL-6 was detected in conditioned media following TNFα treatment. The ability of three different TZDs (ciglitazone, rosiglitazone and troglitazone) to modulate cytokine expression was then investigated. All three TZDs increased TNFα-induced IL-1α or IL-1β mRNA expression, and ciglitazone and troglitazone (but not rosiglitazone) significantly increased IL-6 mRNA and secretion. Our data provide important insights into the regulation of proinflammatory cytokine expression in human cardiac fibroblasts, and suggest that the beneficial effects of TZDs on the myocardial remodelling process are not due to inhibition of TNFα-induced IL-1 or IL-6 expression by cardiac fibroblasts. Rather, the TZDs appear to exert “pro-inflammatory” effects on these cells, which may raise questions regarding the use of this class of drugs in patients with cardiovascular disease.