The value of residual volume, intrathoracic gas volume, functional residual capacity and total lung capacity obtained by the method of body plethysmography and helium dilution method was correlated by single-breath diffusing capacity. These parameters were correlated in 22 patients who had had complete functional diagnostics of the lung: spirometry, flow volume curve, body plethysmography and single-breath diffusing capacity. All findings were within normal limits, which was a condition for selection of patients for our group. Statistically significant differences (p = 0.01) of t-test were established between residual volumes by the plethysmographic and helium dilution method of single-breath diffusing capacity. The residual volume obtained by body plethysmography showed higher values for 526 ml, or 30%. The intrathoracic gas volume showed higher values for 682 ml, or 17%. Inspite of the fact that values of RV obtained by two methods had statistically significant differences, they were still within normal limits, if presented as percent of the predicted value.