<b><i>Background/Aims:</i></b> In term subjects, fat mass (FM) is positively associated with leptin, whereas studies in preterm infants show conflicting results. However, none of these studies measured FM by dual-energy X-ray absorptiometry (DEXA). This study aims to relate FM measured by DEXA in relation to leptin and growth in preterm infants. <b><i>Methods:</i></b> In 139 preterm infants, weight (kg) and length (cm) were measured at birth, term age, and 6 months' corrected age (CA). FM (kg), measured by whole-body DEXA, and leptin (µg/l) were measured at term age and 6 months' CA. <b><i>Results:</i></b> At term age and 6 months' CA, FM was associated with leptin (β = 1.94, 95% CI: 1.51-2.36, and β = 0.37, 95% CI: 0.26-0.48, respectively; p < 0.001). Gain in weight standard deviation score (SDS) between term age and 6 months' CA was associated with FM and leptin at 6 months' CA (β = 0.24, 95% CI: 0.18-0.30, and β = 0.25, 95% CI: 0.16-0.33, respectively; p < 0.001). <b><i>Conclusion:</i></b> In preterm infants, FM measured by DEXA is associated with leptin, which indicates that leptin is a marker of body FM during the first 6 months after term age. Gain in weight SDS between term age and 6 months' CA results in higher FM and higher leptin at 6 months' CA.