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We utilized a standardized in vitro method which employs transmission electron microscopy to monitor the degree of surface activation (cytoplasmic spreading) and amount of aggregation displayed by platelet populations from 314 patients with one of five distinct rheumatic diseases and from 72 normal subjects. The percentage of patients in each group whose platelet populations were hyperactive was as follows: polymyalgia rheumatica, 75 percent; scleroderma, 65 percent; primary gout, 61 percent; rheumatoid arthritis, 57 percent; and degenerative joint disease, 40 percent. Pair-wise contrasts performed after an analysis of variance suggest the following differences and similarities: (1) the mean differential platelet count of the normal subjects differed from that in each disease state; (2) the platelet responsivity in patients with degenerative joint disease most closely resembled that in normal subjects; (3) the platelet response in polymyalgia rheumatica plus temporal arteritis was the most abnormal; and (4) platelet response in scleroderma, rheumatoid arthritis, and gout closely resembled each other. The increased platelet response in vitro may reflect the in vivo presence of disease-related "risk factors" (hyperuricemia, immune complexes, and atherosclerosis). Those patients with "triggered" platelet populations may be appropriate candidates for antiplatelet therapy.