Krypton 81m has many advantages for paediatric lung imaging, although the proper interpretation of ventilation and perfusion lung scans requires the quantitative analysis of the steady state images and clearance curves. Both the theory and practice are described and illustrated by means of 11 paediatric cases aged from 18 days to 14 years. It was possible to calculate the regional distributions of ventilation and perfusion and the normalized ventilation-perfusion (V/Q) ratios. Of greater functional significance was the detection of differences between the clearance rates of inhaled and infused 81Krm within single regions, since such differences reflected the dispersion of V/Q ratios amongst lung units within individual regions.