The influence of a 3-week vegetarian diet and fasting on serum concentration of peroxides, lipids, apolipoproteins, and plasma fibrinogen was studied in ten middle-aged fibromyalgia/fibrositis patients (eight women, two men). Mean serum peroxide concentration (estimated as thiobarbituric acid reacting substances) was reduced from 3.60±0.14 to 2.82±0.15 umol/l (p=0.01) and plasma fibrinogen from 3.33±0.25 to 2.74±0.15 g/l (p=0.02). Serum total cholesterol fell from 6.61±0.50 to 4.83±0.35 mmol/l (p<0.0001), apolipoprotein B from 1.77±0.14 to 1.31±0.11 g/l (p<0.0001), and apolipoprotein A from 1.41±0.09 to 1.23±0.05 g/l (p=0.03). High density lipoprotein cholesterol concentration also decreased somewhat (from 1.26±0.09 to 1.07±0.04 mmol/l,p=0.03) An atherogenic index, reflecting the balance between low and high density lipoproteins, was reduced by 31% (from 5.74±0.79 to 3.97±0.60,p=0.02). The results suggest that vegetarian diet/fasting may have a beneficial influence on the concentration of serum peroxides and plasma fibrinogen concentration, and on the serum level of several lipoprotein-related coronary risk factors.