Interleukin-6 (IL-6) has been shown to be released by cultured human meningioma cells and may be a positive or negative regulator of tumour growth. IL-6 protein and mRNA levels have been examined in a series of meningiomas. In 14 cases, the results are compared with the effects of IL-6 and dexamethasone on growth and IL-6 secretion in vitro. Tumours with the highest in vivo IL-6 mRNA expression also showed maximum induction of IL-6 and increased cellular proliferation on IL-1 stimulation in vitro. Dexamethasone decreased the IL-1-stimulated IL-6 release in all cases. Meningiomas which had little or no IL-6 message were refractory to IL-1 control of IL-6. Remarkably, these formed the group of meningiomas that increased their growth rate in response to dexamethasone.