Geometries of platelets in citrated PRP obtained from normal donors (17) and donors (5) with a hereditary dominant giant platelet syndrome, herein referred to as "Montreal platelet syndrome" (MPS), are compared. The measured geometric axial ratio (rp = thickness/diameter) is used to classify platelet morphologies into three groups: discocytes (rp less than 0.5), disco-echinocytes (rp = 0.5 to 0.9), sphero-echinocytes (rp greater than 0.9). MPS discocytes are normal sized; however, MPS sphero-echinocytes and disco-echinocytes have mean volumes approximately two times larger than normal. It is demonstrated that these larger-than-normal sized MPS platelets can be produced directly from MPS discocytes by treatment with agents known to induce platelet shape change (adenosine diphosphate, thrombin, and incubation at 4 degrees C). Treatment of platelets obtained from normal donors which have been resuspended in MPS PPP and ADP or incubation at 4 degrees C causes the formation of normal-sized disco-echinocytes and sphero-echinocytes. The diameters of MPS disco-echinocytes are identical to the diameters of MPS platelets observed on peripheral blood smear, whereas those of MPS sphero-echinocytes are approximately 20% lower. It is suggested that the appearance of abnormally large platelets in MPS is related to a defect in the mechanism which regulates platelet size and shape during shape change.