53 patients with hyperthyroidism have been analyzed with special reference to therapeutic response to radioactive iodine(131I) treatment. Mean effective half-life, 24 hour uptake rate and radiation dose of 131I in hyperthyroid patients included in this study were respectively. 1) Mean effective half-life of 131I was 4.7¡¾1.5 days in the trace dose and 5.0¡¾1.5 days in the therapeutic dose. 2) Mean 24 hour uptake rate of 131I was 72.7¡¾11.1% in the tracer dose and 73.4¡¾12.3% in the theapeutic dose. 3) Mean radiation dose of 131I was 5319¡¾2648 RAD as predicted and 5692¡¾2843 RAD as actual. A single dose of radioactive iodine treatment was satisfactory in 34 patients (radioiodine resistant) A radioiodine resistant group of patients with hyperthyroidism was distinctively characteristic in the following aspects. 1) Mean thyroid weight calculated in the resistant group (63.9¡¾14.0gm) was significantly (p<0.01) greater than that of the sensitive group (46.6¡¾13.3 gm). 2) Mean 24 hour uptake rate of the tracer dose in the resistant group (67.3¡¾10.7%) was significantly (p<0.05) lower than that of the sensitive group (75.7¡¾10.5%). 3) Mean 24 hour uptake rate of the therapeutic dose in the resistant group (68.5¡¾13.7%) was significantly (p<0.05) lower than that of the sensitive group (76.1¡¾10.9%). 4) Mean predicted radiation dose of 131I in the resistant group (3,684¡¾1,745 RAD) was significantly (p<0.01) lower than that of the sensitive group (6,232¡¾2,683 RAD). 5) Mean actual radiation dose of 131I in the resistant group (4,100¡¾1,691 RAD) was significantly (p<0.01) lower than that of the sensitive group (6,582¡¾3,024RAD). 6) No significant difference was detected in terms of effective half-life of 131I among the groups (p>0.05). 7) The average mean % difference of effective half-life, uptake rate and radiation dose measured following the tracer and therapeutic dose of 131I were not statistically significant (p>0.05). Therefore effective half-life, uptake rate and radiation dose of the therapeutic dose of 131I were readily predictable following the tracer dose of 131I. 8) It is concluded that the possibility of resistance to radioactive iodine treatment may be anticipated in patients with thyroid gland large in size and compromised 131I uptake rate.