There have been few studies on the effects of a fast on clinical laboratory data in Japanese. We studied twelve women with rheumatoid arthritis who were not taking any medicine and stayed in the Koda hospital for a diet which lasted 55 days. They basically took a 1200 kcal vegan diet and undertook a 3-5-day fast three times. The clinical laboratory data obtained before and after the second fast (day 27-day 31) were compared. Average body weight decreased by 1.5 kg. There were no changes in CRP. Rapid turnover proteins such as alpha 1 and beta 2-microglobulin decreased, whereas albumin, IgG, IgA and IgM increased. HDL-C increased without a change in LDL-C or triglycerides. Free T3 decreased and free T4 increased, while TSH did not change. The increases in albumin, Ig, HDL-C and free T4 were not consistent with the results of previous studies. This difference may have been due to the low calorie vegan diet before the fast.