Xylem development and crown growth of eight 7.5 m tall loblolly pine saplings was followed through one growing season. Periodically induced xylem wounds allowed time-related development of cell and ring dimensions to be determined. Periodic measurement of the leader and all major first-order branches allowed rates and indices of crown growth to be determined. Crown growth and annual ring development were compared in linear regressions to determine effects of crown activity on xylem development. Cell diameter and wall thickness varied directly with a crown growth index expressing a combination of shoot and needle elongation rate, distance between shoots and points on the stem, and cambial age at these points. Ring dimensions appeared dependent on a cumulative expression of shoot and needle growth rate, but no effect related to cambial age was involved at any stem level. Forest Sci. 22:247-260.