Acromegaly is frequently associated with the presence of thyroid disorders, however the exact prevalence is still controversial. An Italian multicenter study was performed on 258 patients with active acromegaly (high levels of IGF-I and lack of suppression of serum GH levels below 2 microg/l after an OGTT). The control group was represented by 150 patients affected by non-functioning and PRL-secreting pituitary adenomas. Two hundred and two out of 258 acromegalic patients (78%) were affected by thyroid disorders with a significantly higher prevalence with respect to the control group (27%, p<0.0001). One hundred and three patients presented (39.9%) non-toxic nodular goiter, 46 (17.8%) non-toxic diffuse goiter, 37 (14.3%) toxic nodular goiter, 1 toxic diffuse goiter (0.4%), 12 (4.6%) Hashimoto's thyroiditis, 3 (1.2%) thyroid cancer. Two patients presented a co-secreting TSH pituitary adenoma. Thirty-six patients had been previously treated for various thyroid abnormalities. Among the 222 acromegalic patients never treated for thyroid disorders thyroid ultrasonography was performed on 194 subjects. Thyroid volume in patients with thyroid abnormalities was 28+/-17.5 ml (median 23) while it was 10.8+/-3.6 ml (median 10) in patients without thyroid disorders (p<0.0001). Thyroid volume was correlated with the estimated duration of acromegaly (r=0.7, p<000.1), but not with age or with serum GH, IGF-I and TSH concentrations. Thyroid volume was higher in acromegalic patients than in the above control population (23.5+/-16.9 ml vs 13.9+/-12.8 ml, p<0.0001). In 62 acromegalic patients 101 fine-needle biopsies of thyroid nodules were performed; 7 nodules were suspicious and the patients were submitted to thyroid surgery: papillary thyroid carcinoma was found in 3 patients. In conclusion, in a large series of acromegalic patients an increased prevalence of thyroid disorders (78%), particularly non-toxic nodular goiter, has been observed. Thyroid volume, evaluated by ultrasonography, was correlated to the estimated duration of acromegaly. Finally, the prevalence of thyroid carcinoma was slightly increased than in the general population.