Forty-six benign and malignant tumours and tumour-like lesions of the thyroid were analysed for microsatellite instability (MI) at eight loci, mapping to four different chromosomes, 7 lesions (15%) displayed MI at one or more loci, including 2/13 nodular goitres, 2/15 follicular adenomas, 2/12 papillary carcinomas and 1/4 follicular carcinomas. Two benign and one malignant lesion among the seven unstable cases exhibited this phenotype at three or more loci. We found no mutations in the mismatch repair gene, hMSH2, in the seven affected cases, after screening all the exons by CDGE mutation analysis. At variance with the data on record, these results indicate that, despite being relatively infrequent, MI does occur not only in thyroid carcinomas but also in benign lesions (goitres and follicular adenomas of the thyroid).