The validity of total body potassium (TBK) measurement in estimating fat mass and fat-free mass (FFM) in GHD adults was assessed by comparison with the reference technique of dual energy x-ray absorptiometry (DEXA). The TBK and FFM values determined by DEXA were used to calculate the potassium concentration per kg FFM in GH-deficient (GHD) adults and compared with standard values for normal subjects of 59.6 mmol for females and 66.4 mmol for males. There were considerable differences between predicted and measured TBK values for both males (3972 vs. 3577 mmol; P < 0.001) and females (2526 vs. 2277 mmol; P < 0.001). Similarly, the estimation of FFM and fat mass by TBK measurement was significantly inaccurate for both sexes compared to values determined by DEXA. These discrepancies may be accounted for by the lower calculated potassium concentrations compared with standard values for both males (56.2 vs. 66.4 mmol; P < 0.001) and females (53.1 vs. 59.6 mmol; P < 0.001). These observations suggest that caution should be exercised in the interpretation of TBK in GHD adults, and the reduced potassium concentrations would alleviate inaccuracies in the estimation of body composition. Secondly, the decreased intracellular potassium concentration of GHD adults may account for the decreased muscle strength and ease of fatigueability seen in GHD adults.