We measured interleukin- 1 beta (IL- 1 beta) and tumour necrosis factor- alpha (TNF-alpha) in the vitreous humour and serum of patients with proliferative diabetic retinopathy (PDR), in order to determine the role of these cytokines in the pathogenesis of the disease. Vitreous and serum samples were collected from 21 patients with PDR who were undergoing pars plana vitrectomy. Control vitreous samples were obtained from cadavers and control serum samples from healthy subjects. The cytokines were measured by enzyme-linked immunosorbent assay. Vitreous IL- 1 beta and TNF-alpha concentrations in patients with PDR exceeded those of controls (P < 0.05), as did serum IL- 1 beta and TNF-alpha. We suggest that increased vitreous IL- 1 beta and TNF-alpha levels may play a significant role in the pathogenesis of PDR, which features abnormal cell proliferation and neovascularisation.