Human whole body counters have many practical applications to diagnosis in clinical medicine. Human counters in NIRS have been employed in a number of clinical tracer studies for measuring the whole body retention and absorption of very low levels of various isotopes and labelled compounds, such as NaI-131, Sr85, Na-22, Cr-51 labelled red cell, Fe-59 and Co-60 labelled VB12. The results obtained illustrate the potentialities of whole body counting in clinical diagnosis. Natural radioactive K-40, which is said to be approximately 0.0119% of the total potassium in the human body can be measured without any isotope ingestion using a human counter. Measurement of the total body potassium gives an important information on physiological and pathological body conditions as muscular development, lean body mass to fat ratio and electrolyte balance. The study was designed to estimate of the total body potassium in the patients with periodic paralysis and to know its relation to gross body composition. There was no significant difference between the lean body mass content of potassium in the periodic paralysis patients and that in the control subjects. There was a poor correlation of the total body potassium with gross body weight and a much better correlation in the both groups if the total body potassium is related to the lean body mass. Human whole body counter may play an important role in establishing the diagnosis of the neuromuscular diseases associated with potassium deficiencies and some hormonal diseases which cause disturbance of fat metabolism.