OBJECTIVE: To study the relation between body mass index (BMI) and age on the one hand and total number of human (pre-) adipocytes and preadipocyte differentiation capacity on the other hand. SUBJECTS: In total, 189 women undergoing surgical mammary reduction, age range 16-73 y, BMI range 19.7-39.7 kg/m(2). MEASUREMENTS: Differentiation of preadipocytes in primary culture was assessed by morphological criteria, and determination of glycerol-3-phosphate dehydrogenase after stimulation of the cells by standardized adipogenic conditions containing isobutyl-methylxanthine, troglitazone or both compounds. The total number of stromal cells (ie preadipocytes) and fat cells per gram of adipose tissue and per body as well as mature fat cell volume were calculated from isolated stromal cells and adipocytes, respectively, and anthropometric measures. RESULTS: BMI correlated positively to age, mature fat cell size and total number of adipocytes and stromal cells per body (r varying from 0.22 to 0.54, each P<0.05). In contrast, BMI correlated negatively to the number of adipocytes and stromal cells per gram of adipose tissue and the capacity of preadipocytes to differentiate (r varying from -0.20 to -0.37, each P<0.05). No significant correlation was observed between BMI and the ratio of stromal cells to adipocytes. The sample was also divided into three groups: BMI <25 kg/m(2) (lean), BMI 25-29.9 kg/m(2) (overweight) and BMI >/=30 kg/m(2) (obese). The overweight group showed a larger fat cell size but no increase in total fat cell or stromal cell number when compared to the lean subjects. The obese subjects showed larger stromal and fat cell numbers when compared to the lean subjects. Age did not independently correlate to the number of stromal cells or adipocytes per gram of adipose tissue or total body, nor with the capacity of preadipocytes to undergo differentiation and the ratio of stromal cells to adipocytes. CONCLUSION: There seems to be a constant ratio between the number of adipose tissue stromal cells and adipocytes independently of BMI and age in humans. During adipose tissue expansion, there seems to be both a continuous increase in fat cell size, and in stromal cell and adipocyte number, but the increase in fat cell size apparently precedes the increase in fat cell number. The differentiation capacity of the stromal cells appears to decrease with increasing BMI.