Hashimoto's thyroiditis (HT) and lymphoma are sometimes difficult to distinguish between. Moreover, lymphoma sometimes develops in a thyroid gland from pre-existing HT. Open- or large-needle biopsy usually distinguishes between them; the specimen may be examined histologically and subjected to immunohistochemistry. Another possible method of examination is fine-needle aspiration biopsy (FNAB). The cells obtained may be evaluated cytologically, and subjected to flow cytometry, using various antibodies. In this study, anti-kappa and anti-lambda antibodies are especially important, as a gross predominance of kappa or lambda B lymphocytes infiltrating the thyroid is evidence for a B cell monoclone. In this study, 15 patients were selected because of their rapidly growing goitres. They all underwent FNAB. Five had cytology typical of HT, and no evidence of monoclonality on flow cytometry. They were diagnosed as HT without further histopathology. The remaining 10 patients had cytology suspected of lymphoma, or evidence of monoclonality on flow cytometry, or both. These patients underwent open- or large-needle biopsy. Only three of them were diagnosed histopathologically as lymphoma; the other seven were diagnosed histopathologically as HT, making 12 cases of HT in all. Five of these 12 cases, and one of the three cases of lymphoma showed flow cytometrical evidence of monoclonality; thus evidence of monoclonality from FNAB, while interesting, does not necessarily serve to differentiate between HT and lymphoma. Furthermore, the immunohistochemical assessment of monoclonality did not correlate with the flow cytometrical assessment. Follow-up evidence will be required to discover whether those patients with a B cell monoclone in their HT are the ones who develop a lymphoma.