In the past 6 years the author has treated 51 cases of subacute thyroiditis. Their data are summarized below:The disease was confirmed by Silverman's needle biopsy of the thyroid in 36 them, and the others were diagnosed by typical clinical symptoms, characteristic results of thyroidal function test and accelerated red cell sedimentation rate its clinical course.Of the total cases, 4 were males and 47 females, and the ages ranged 19 to 62 years, the thirties and forties together comprizing 40 cases (78.4%). As for clinical symptoms, goiters were observed in all of the total 51 cases, followed by local tenderness (96.1%), referred pain (92.2%), fever (90.2%), general lassitude (86.3%), headache (80.4%), palpitation (72.5%), body weight loss (62.7%), finger tremor and abnormal perspiration (60.8% each), and inflammation of the upper respiratorytract (41.2%). It was interesting that 27 out of 37 female cases (71.1%) had hypo-or oligomenorrhea, whereas in convalescence 29 of 38 (76.3%) had hyper-or polyrnenorrhea. Goiters of all the cases were slight or moderate.Results of clinical and laboratory examinations in acute phasc: Ranges for 51 cases; body temperature 37.0-40.1°C (Mean ± Standard Deviation 38.4 ± 0.7°C); pulse rate permin. 72-162 (95.4 ± 21.5); BMR-1.6-+62.1% (+20.7 ± 13.2%); serum PBI 3.8-16.8γ/dl (6.9 ± 2.4γ/dl); thyroidal I131 uptake 0.2-10.8% (2.9 ± 2.8%); red cell sedimentation rate per hr. 20-142mm (73.8 ± 34.5mm). Results of examinations of 10 cases at 1 to 2 weeks after the onset: BMR +15.4-+32.8% (Mean+25.3%); serum PBI 6.1-16.8γ/dl (Mean 10.2γ/dl); I131 uptake rate 0.2-5.1% (Mean 1.2%); red cell sed imentation rate per hour 73-132mm (Mean 105.2mm). Serum cholesterol for 11 cases 140-225mg/dl (188.9 ± 26.5mg/dl).Red cell count for 37 cases 300-523×104/cmm (405.3+56.0) ×104, 7 giving below 350×104/cmm (18.9%); Sahli's, value 57-105% (81.1±7.3%), 7 giving below 70% (18.9%); white cell count 3, 200-13, 200/cmm (6, 724±2, 244/cmm), 18 giving below 6, 000/cmm (48.6%) and 11 above 8, 000/cmm (29.7%) and in 4 of these 11 cases above 10, 000/cmm (10.8%). Blood eosinophile cell count was low in all the cases. Serum iron for 10 cases 66-143γ/dl (95.1±28.8γ/dl), 5 giving moderately low levels; UIBC 268-394γ/dl (316.1±40.3γ/dl), 6 giving above 300γ/dl; serum copper for 3 cases 148-154γ/dl (Mean 154.0γ/dl), all giving high values.Serum protein tractions were determined by the Tiselius method with 20 cases, and the results were in agreement with past reports except that spike was observed in the descending fraction of β-globulin. As to liver function, there was abnormality in Takada-Jezler reaction, cobalt-reaction, cadmium reaction and CCF test, revealed delay in the recovery from hyperglycemia in 6 of 12 cases, and insulin tolerance test showed abnormality in 10 of 16 cases. Abnormality in Thorn's test, serum Na and serum K, and decrease in urinary 17 KS were observed in a few cases. Five cases in acute phase showed positivity in Shichijo's xerogel-figure, and skin electric resistance indicated the type of hyperthyroidism.Precipitation test was performed on 10 patients with subacute thyroiditis using human thyroglobulin as antigen, and the antibody was demonstrated in the serum from 6. Also skin test was performed with the above mentioned antigen, and redness could be demonstrated in 6 of 10 patients with this disease.