We have correlated the morphological features of 30 human thyroid carcinomas with the cellular localisation of insulin-like growth factor 1 (IGF-1) mRNA and IGF-1 receptor peptide using in situ hybridisation with digoxigenin-labelled oligoprobes and immunohistochemistry. Four of the five follicular carcinomas studied showed a consistent, uniform, strong positivity for IGF-1 mRNA in tumour cells compared with weakly positive surrounding normal follicular tissue and negative stroma. The majority of papillary carcinomas showed weak to moderate epithelial positivity for IGF-1 mRNA and negative stroma. Immunohistochemistry for IGF-1 receptor showed moderate positivity confined to the tumour epithelial cells in both follicular and the majority of papillary carcinomas. However, in a subgroup of papillary carcinomas characterised by a diffuse stromal lymphoid infiltration (n = 5), the stromal cells showed a much stronger reactivity for IGF-1 mRNA than the tumour or background thyroid, and the tumour cells showed a uniformly high level of immunoreactivity for IGF-1 receptor. These results are compatible with the growth of the papillary carcinoma in these cases being the result of a symbiotic relationship between the stromal lymphoid cells and the tumour epithelium with the lymphoid cells responding to an antigen produced by the tumour cells and the tumour cells responding to a growth factor produced by the lymphoid infiltrate. We suggest that this mechanism may be important in other tumours regularly associated with a widespread lymphoid infiltrate.