To determine whether cytokines could have a role in the development of insulin-dependent diabetes mellitus (IDDM), we measured serum levels of cytokines derived from T helper 1 (interleukin-2 and interferon-γ), T helper 2 (interleukin-4 and inter-leukin-10) lymphocytes and macrophages (tumour necrosis factor-α, interleukin-1 α and interleukin-1 Β) in patients before and after the onset of IDDM. Recently diagnosed IDDM patients had significantly higher levels of interleukin-2, interferon-γ, tumour necrosis factor-α and interleukin-1 α than patients with either long-standing IDDM, non-insulin-dependent diabetes (NIDDM), Graves' disease, or control subjects (p<0.05 for all). Compared with control subjects, patients with long-standing IDDM and those with NIDDM had higher interleukin-2 and tumour necrosis factor-α levels (p<0.01 for all). Interleukin-4 and interleukin-10 were detectable in sera of patients with Graves' disease only, while interleukin-1 Β was not detectable in the serum of any control or test subject. To investigate whether high cytokine levels precede the onset of IDDM, we studied 28 non-diabetic identical co-twins of patients with IDDM, followed-up prospectively for up to 6 years after the diagnosis of the index. Levels of tumour necrosis factor-α and interleukin-1 α were elevated above the normal range more frequently in the eight twins who developed diabetes than in those 20 who did not (p<0.005). Analysis of T helper 1 and T helper 2 profiles of the twin groups did not reveal a clear difference between prediabetic twins and twins remaining non-diabetic. These results support the notion that T helper 1 lymphocytes may play a role in the development of IDDM. This is associated with release of macrophage-derived cytokines, which is also a feature of the prediabetic period. The lack of evidence of a dominant T helper 1 profile of cytokine release before diabetes onset suggests that additional events, activating this arm of the cellular immune response, are required in the immediate prediabetic period.