Erythroblasts from a patient with congenital dyserythropoietic anemia type III (CDA III) were observed by time-lapse phase-contrast cinemicrography. The erythroblasts were compressed with a new mechanical device; this procedure enhanced the visualization of their structures. Soret band micrography was carried out simultaneously to assess cytoplasmic hemoglobinization. The proliferation and differentiation course of the majority of CDA III erythroblasts was similar to that of normal erythroblasts. The formation of dysplastic erythroblasts showed two patterns of progression: (1) Usually, the process began with the appearance of a well-demarcated clear area in the nucleus of immature erythroblasts. This clear area reappeared in the progeny of the immature erythroblasts, at an increased incidence and in increasing size and rigidity. It finally came to occupy most of the nucleus, compartmentalizing it. The maturing cells displayed vigorous movement and progression of hemoglobinization and the nuclei became lobular; they grew to the cell size at which most CDA III erythroblasts divide, but did not do so. (2) Less frequently seen were abnormalities in mitosis and cellular behavior (e.g., tri- or bi-polar mitosis and subsequent fusion of daughter cells) or rapid growth of one daughter cell with concomitant shrinkage of the other.