Bronchial asthma and RA patients receiving gold therapy were examined to study the mechanisms of efficacy and side effect of gold therapy. The results are briefly summarized as follows. 1) Seventy percent of asthma patients were treated succesfully by gold therapy. Most of these patients showed clinical improvement at the time the total accumulated dosage of gold compounds was about 370mg and the plasma concentration of gold was about 260 g/dl. 2) At 2 months after beginning gold therapy, transient increases of urinary 17-KGS values were found in asthma patients. This finding suggested that gold compounds may stimulate the hypothalamo-pituitary-adrenal axis. 3) Gold salt has an unknown factor that lowers the serum IgE levels of asthma patients. This unknoun factor probably controls the effective mechanisms of gold salt. 4) Elevated serum IgE levels were frequently found in asthma and RA patients who showed side effects to gold therapy. 5) The mechanisms of side effects of gold therapy were classified into 3 types: immediate and delayed type allergy and toxicity.