SummaryIn vitro megakaryocyte colony formation was studied in 25 patients with chronic idiopathic thrombocytopenic purpura (ITP). and compared to 16 controls. A significantly lower number of megakaryocyte progenitors (CFU-MK) was observed in patients (10·8 ± 124 CFU-MK per 105 bone-marrow non-adherent mononuclear cells (NAMC), compared to controls (23·5 ± 15·4 CFU-MK): P<0·002. Only six patients had a normal number of CFU-MK. The number of CFU-MK was not correlated to initial platelet count (P= 0·35). These findings indicate that in vitro CFU-MK could be of interest in the diagnosis of chronic ITP, and also could be predictive for chronicity in patients with acute ITP.