The authors investigated basal and glucose stimulated (50 g by mouth) IRI values in women with normal weight and obese women (58) under conditions of balanced body-weight and after its reduction. The body composition was determined (from body density), and from specimens of subcutaneous abdominal adipose tissue also the size of fat cells and their total number. In obese women significantly higher IRI levels (basal and stimulated) were found as compared with controls and these values had a marked tendency towards normalization after reduction of body weight. The authors found significant relations between IRI values and the degree of obesity, fat content and lean body mass. The closest correlation was found between the stimulated IRI values and Broca's index (r = +0.8227). Between the loss of body-weight and body fat and between changes of IRI in obese subjects no significant relations were found. Investigation of the relationship of IRI and the size and total number of fat cells revealed marked associations between basal values and the sum of stimulated IRI values and the size of the fat cell. Relations between IRI and the total number of fat cells were not significant. When investigating the relationship between the incidence of obesity in the family and IRI values it was revealed that the group of obese women with obese mothers, as compared with the group who had neither parent obese, had a significantly higher basal IRI value and IRI value after stimulation with glucose during the 120th and 180th minute, the higher basal value in the group with an obese father was not significant. After weight reduction the differences between basal and stimulated IRI values were not significant.