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The activity of a supposedly heteromeric complex (C4/5) of butyrylcholinesterase (BChE; EC was analyzed in relation to body mass index (BMI). The activities of the C4/5 and COF (other molecular forms) bands in CHE2 C5– (n = 447) and CHE2 C5+ (n = 88) individuals were quantified by densitometry. Since the absolute activity of C4/5 (AC4/5) showed the highest correlation coefficient with weight in the CHE2 C5– phenotype when compared to the other BChE activity variables (total, relative C4/5, and absolute COF), this variable was used for the classification of 51 CHE2 C5– individuals into three groups (low, average, and high), paired by sex, age, and ethnic origin. The low AC4/5 group was found to present a significantly (p < 0.0001) lower mean BMI (22.9) than the other groups (average = 25.2, and high = 26.3). In the CHE2 C5+ individuals no statistically significant standardized regression coefficient was verified between BMI (dependent variable) and the C4/5 band activities. These data show that the behavior of the C4/5 band in relation to BMI differs between the CHE2 C5– and CHE2 C5+ phenotypes. While the C5 band of the CHE2 C5+ individuals is negatively associated with fat storage in the adipose cells, the present data show that the C4/5 band is positively associated with this storage in the CHE2 C5– phenotype.