We have adopted OK-432 as a sclerosing agent in the treatment of cystic predominant thyroid nodules and have analyzed our findings to assess the efficacy of intralesional instillation of OK-432. From 1992 through 1993, 48 patients with recurrent or progressive cystic thyroid nodules after 2 or 3 aspirations alone, and whom were cytologically negative for malignancy, were used for this study. The OK-432 solution was prepared by dissolving 0.1 mg of OK-432 in 2 ml of physiologic saline and it was instilled in the amount of 1/10-to-1/5 of the aspirated cystic fluid. A repeated course of therapy was given up to 3 times when sufficient resolution was not obtained 4-to-6 weeks after treatment. The mean number of treatment sessions per patient was 1.5. Throughout the follow-up period from 30-to-45 months (mean, 38 months), 32 (66.7%) patients experienced an almost complete disappearance (< 0.05 cm in diameter) of the cystic lesion, and 12 (25%) patients responded partially by having it decrease by more than half (> 0.5 cm in diameter) of the initial cyst size, and none of these patients required further treatment. The remaining 4 (8.3%) patients showed insufficient resolution despite 3 courses of therapy and 2 of these patients underwent thyroidectomy, in which the lesion proved benign in both cases. All of the patients tolerated the sclerotherapy well. No significant local complications attributed to this treatment were observed. However, a low-grade fever was observed in 26 (54.2%) patients for 2 to 5 days after instillation, which subsided with symptomatic treatment. On the basis of our experience, OK-432 sclerotherapy appears to be safe, simple and effective, and can be a useful alternative treatment for cystic thyroid nodules.