Fat mass is increased in renal transplant (RTx) patients, which may have untoward metabolic and cardiovascular effects. The influence of steroids on body composition (BC), resting energy expenditure (REE), and substrate oxidation rates was assessed in stable RTx patients in a cross-sectional design. Also, the relation between physical activity and nutrient intake, respectively, and body composition was studied.77 RTx patients (42 males, 35 females) were studied. Twenty-one patients were on 10 mg and 27 patients on 5 mg maintenance steroid dose; 29 patients were receiving steroid-free immunosuppression. Assessed were BC (DEXA, anthropometry), REE and substrate oxidation (indirect calorimetry), physical activity (Baecke questionnaire), and nutrient intake (dietary records).BC was not different between the 0-, 5-, and 10-mg steroid group, and no relationship existed between cumulative dose of steroids and BC. REE and substrate oxidation also did not differ between the various groups, apart from a small increase in glucose and decrease in lipid oxidation in female patients using 5-mg steroids. Especially in females, leisure time physical activity was positively related with the percentage lean body mass (r=0.571, P=0.004) and inversely related with fat mass (r= -0.588, P=0.003). Nutrient intake and BC (corrected for physical activity) were not related.No relation was observed between daily and cumulative steroid dosage and BC and between daily steroid dose and REE and substrate oxidation in RTx patients. Especially in female patients, physical activity level and the percentage of lean body mass concluded and body fat were significantly related.