We have studied the smoking habits in a group of patients with hyperthyroidism caused by Graves' disease. One hundred and seventy-one patients were randomized to different forms of treatment for hyperthyroidism and the outcome of this study concerning Graves' ophthalmopathy has been described previously. There were 89 smokers (52%) and 82 non-smokers (48%). Among the smokers there were 32 patients (19%) who developed ophthalmopathy or deteriorated in an already present ophthalmopathy and 57 patients who did not (33%), whereas among the non-smokers, 14 (8%) had ophthalmopathy during the study and 68 (40%) did not (p = 0.006). We could not obtain statistical significance when trying to demonstrate that smoking is a risk factor for the development of ophthalmopathy after treatment for hyperthyroidism. It was noted that smokers had higher pretreatment levels of thyrotropin-receptor antibodies (p = 0.027). In conclusion, these results support the previously described association between smoking and Graves' ophthalmopathy.