Diameter and height distributions forPinus radiataD. Don trees grown from seed lots representing a range of genetic improvement were compared at midrotation (age 14 or 15) in seven large-plot trials at six sites. In one of the trials, comparisons were made at year 5 and annually from age 8 to 16. These are the first data from plantation conifers comparing tree size distributions of commercially planted seed lots. Differences among seed lots for quadratic mean diameter and mean height were statistically significant and generally reflected the expected level of genetic improvement. Standard deviation, skewness, and kurtosis were not significantly different among seed lots. However, diameter distributions of higher rated seed lots sometimes appeared very slightly more skewed to the right and flatter than the lower rated seed lots, a similar tendency observed as stands age. Models used to predict diameter distribution from stand parameters are not likely to require modification for genetically improved seed lots.