We studied the platelets of 8 patients with myelodysplasia aged 49-77 years, using both ultrastructural and functional techniques. Five of the 8 patients were classified as having refractory anaemia, and 3 as refractory anaemia with excess of blasts (RAEB). Electron microscopically, the myelodysplastic patients had in addition to normal ones, platelets containing significantly less alpha-granules. In part of these hypogranular platelets, the dense tubular system was abundant, but in contrast with normal platelets, it was not dispersed between the other organelles, nor did it form a membrane complex with the open cannalicular system. From a functional point of view, collagen-induced shape change was the most frequently disturbed parameter: there was a total loss of collagen-induced shape change in 5 patients. In 2 patients, there was a complete lack of response to collagen in platelet-rich plasma (both shape change and aggregation); in one of them, there was also a total loss of adenosine triphosphate secretion in response to all inducers tested. After 4 years of follow-up, 5 patients had died, of whom 3 were RAEB patients. An initial complete absence of collagen-induced shape change was found in these 5 patients, while in the 3 patients who were still alive at the end of the follow-up period, collagen-induced shape change was normal in 2 and slightly diminished in one.