The clinical course of ITP in children usually differs from that in adults, yet distincttons by marrow characteristics have not been established. Megakaryocytes (megas) were examined for size, ploidy, maturation and morphology in 8 children and 8 adults with ITP, and 8 normal marrows. Either buffy coat wedge or cover slip squash smears were prepared. (Control values differed for each.) Feulgen-stalned megas (100-300) were examined as previously described. Wright-stained material was also examined. There were no differences in the megas of normal children and adults. In all children with acute ITP, megas were increased in size (mean volume 4X normal), ploidy (medians 2 doublings higher, to 64N) and maturation stage (86% mature vs control 43%). In contrast, In adults with acute or chronic ITP the megas showed no differences In size, ploidy, or maturation compared to controls. Some dissociation of mega ploidy and maturation was seen in adults, but not enough to alter the profile of any one parameter. In conclusion, the megas of acute childhood ITP were different than adults'. Similar enlargement and maturation is seen in animals whose megas are stimulated by injection of anti-platelet serum. The absence of these large, mature megas in adult ITP may reflect the chronicity of this disease In adults. Studying children with chronic ITP with regard to marrow mega characteristics may provide information to help identify those children with acute ITP with a propensity to develop chronic ITP.