Adipose tissue fat cell size and number have been determined in patients with endogenous hypertri‐glyceridemia. In addition to increased plasma triglycerides these patients were characterized, as previously well known, by a decreased glucose tolerance, less frequently an increase in fasting plasma insulin and plasma insulin during glucose tolerance, as well as by moderate obesity. The obesity was due to increased fat cell size rather than fat cell number. In the hypertriglyceridemic patients statistical associations were found between fasting plasma insulin and fat cell size.