We studied a large kindred with nonsyndromic autosomal dominant thrombocytopenia to define the phenotype and used genomic linkage analysis to determine the locus of the abnormal gene. Affected family members are characterized by lifelong moderate thrombocytopenia (mean = 42.7 × 10⁹/L) with moderate propensity toward easy bruising and minor bleeding. Megakaryocytes are present in bone marrow with reduced frequency, and there are no apparent abnormalities of myeloid or erythroid cells. This type of inherited thrombocytopenia has no evident association with hematopoietic malignancy or progression to aplastic anemia. In the past, members of this family have failed therapeutic trials of immunosuppression and splenectomy. In our investigation, we found that affected individuals had normal platelet size compared with unaffected family members and modestly increased thrombopoietin levels. Hematopoietic colony assays from bone marrow and peripheral blood demonstrated that megakaryocyte precursors (CFU-Mk) were dramatically increased in both number and size in affected individuals. Bone marrow cells grown in liquid culture with thrombopoietin failed to develop polyploid cells greater than 8N. Also, electron microscopy demonstrated that megakaryocytes from an affected individual had markedly delayed nuclear and cytoplasmic differentiation. Genome-wide linkage analysis established a single locus for the disease gene on the short arm of chromosome 10 with a maximum 2-point lod score of 5.68 (at θ = 0). By recruiting additional family members, the genomic region was narrowed to 17 centimorgans. We conclude that a gene in this locus plays an important role in megakaryocyte endomitosis and terminal maturation.