Background: Adipose tissue contains complex populations of large and small adipocytes. Changes in fat cell size (FCS) distribution with weight loss are unclear, especially with regards to small adipocytes. The aim was to evaluate changes in FCS distribution in two depots following weight loss and determine the association with fat mass loss and anthropometrics. Methods: Healthy adult volunteers (6 men/4 women; Age 33 ± 11 years; BMI 35 ± 6 kg/m2) underwent DXA and subcutaneous abdominal and thigh fat biopsies before and after 6 weeks of 50% inpatient caloric restriction. Osmium-fixed adipocytes were sized with a Coulter counter. Cell size distributions were fit to nonlinear regression to obtain peak large cell diameter (LCD), nadir diameter (midpoint between large and small populations), and percentage of small cell (cells with diameter < nadir diameter). Results: Fat mass, waist and thigh circumferences decreased (Д = -4.0 kg, Д = -8.1 cm, and Д = -3.8 cm, p's < .0001). Abdominal LCD (Д = -13.2 |im, p = 0.01) and nadir (Д = -7.3 |im, p = 0.03) also decreased. In repeated measures correlation accounting for cell sizes before and after weight loss, abdominal LCD and nadir were associated with fat mass loss (rrm = 0.68, p = 0.02; rrm = 0.63, p = 0.04) and decrease in waist circumference (rrm = 0.70, p = 0.02; rrm = 0.60, p = 0.05). There was a borderline decrease in thigh LCD and nadir (Д = -5.5 im, p = 0.09; Д = -6.1 im, p = 0.11). Thigh LCD and nadir were associated with fat mass loss (rrm = 0.66, p = 0.03; rrm = 062., p = 0.04) but not change in thigh circumference. Percentage of small cells did not change in either depot and was not associated with changes in fat mass. Conclusions: Caloric restriction reduces adipocyte LCD and nadir. These changes are associated with fat mass loss. As waist circumference decreases, abdominal LCD and nadir decreases in tandem. Changes in percent of small cells were not seen. This indicates that larger fat cells should be considered as phenotypic targets for weight loss.