Immunofluorescent demonstrations of platelet-free fibrin deposit in the micro and macro-vascular lesions of maturity onset diabetes mellitus and atherosclerosis by earlier workers called for investigations of fibrino-coagulopathy in the two disorders. The present investigation included the determinations of plasma thrombin time (PTCT), fibrinogen concentration (FC), plasma fibrinogen half life (PFT ½) and plasma fibrinogen degradation product (FDP) in (i) the subjects of the two age-onset disorders and their consanguinous family members, and (ii) juvenile diabetes. The assays were carried out by standard methods. 125Iodine labelled human fibrinogen was used for half life studies. In a study of 525 subjects, very highly significant (p * This study was presented in part at the III Congress of International Society of Thrombosis and Haemostasis, Washington DC, USA, August, 1972.