Results of the long term effects of two schedules of radioine therapy (I131) in 130 toxic multinodular goitre patients were evaluated. Seventy five patients (group I) were treated with low doses and 55 patients (group II) with calculated high doses adjusted for thyroid weight (0.5-1 mu ci/g) and radioiodine uptake. Follow up (mean +/- SEM) was 4.5 +/- 0.4 years and 4.8 +/- 0.6 years respectively (p > 0.1). At the end of followup hyperthyroidism was successfully reversed in 87% (Group I) and 82% (Group II). In group I hypothyroidism was present in 5% of patients while it was 12.5% in group II patients. The total dose per gram of thyroid tissue was not significantly different in both the groups (0.058 mu ci +/- 0.0054 VS 0.073 +/- 0.0054 ci/g.) However in group II the number of I131 administration was significantly lower (1.5 +/- 0.2) than in group I (3.2 +/- 0.4) and the percentage of patients who were adequately treated in Group II with single dose was more as compared in group I (62% in group II versus 49% in group I) Hypothyroidism was reached in a shorter time after treatment in group II (median time 0.8 year in group II Vs 1.1 yrs in group I). Patients with positive thyroid antibodies showed a significant earlier development of hypothyroidism within six months. It is concluded that radioiodine is an effective treatment for toxic multinodular goitre with a significant low incidence of post therapy hypothyroidism in patients treated with low doses as compared to higher doses of radioiodine therapy.