Uninjected postmortem normal lung from 49 children aged 1 hour to 14 years was studied using quantitative morphometric techniques to assess arterial size, number and muscularity, particularly in the respiratory region of lung. Arterial size increased most rapidly during the first 2 months of life, but growth rate remained high during the first 4 years. At all ages, the range of values for the mean external diameter of arteries accompanying peripheral airways was considerable, but marked differences from the normal range were usually associated with a marked difference in stature. Arterial number increased rapidly in the first 2 months, but subsequently arteries multiplied at the same rate as alveoli, and the alveolar:arterial ratio was relatively constant. Mean percentage arterial medial thickness fell quickly during the first 10 days and continued to decrease during the first 3 months of life, after which there was little change. The intra-acinar arteries became more muscular during childhood as they increased in size. Vein muscle wall thickness was low throughout childhood. The normal values given in this paper provide a basis for the evaluation of lung biopsies in childhood.