The influence of heredity factors in Cerebrovascular accidents was investigated by studying the families of 80 patients with a clinical diagnosis of CVA. The frequency of CVA in parents and siblings of these patients was compared with the frequency in the family of the patient's spouse. The frequency of recognized predisposing illnesses to CVA including hypertension, diabetes and heart disease was also studied. The patients and the spouses were excluded from the study population. Analysis of the data obtained on 160 parents and 384 sibs of the proband and on 140 parents and 336 sibs of the spouse revealed a frequency of CVA of 10.7% and 8.6% respectively. This difference was not statistically significant. However, when the sibs and parents were analyzed separately the difference between the sibs was significant (p < 0.025), suggesting the possibility that a small added risk of CVA existed for certain close relatives of a CVA victim. Besides an inherited tendency to CVA, other factors were considered to account for the difference in frequency of CVA. Age, family size and differential reporting of illness failed to account for the difference. However, both hypertension and heart disease occurred with greater frequency in the sibs of the patient. When patients with these predisposing illnesses were excluded and those with CVA alone were compared, it was found that relatives of the patient and the spouse had essentially the same frequency (3.1% and 3.2% respectively). Moreover, hypertension and heart disease were significantly more common in the relatives of the proband. The excess of CVA in the sibs of the proband could, therefore, have been due to an excess of predisposing illnesses such as hypertension and heart disease, and no independent inheritance of CVA was demonstrated. In the absence of certain predisposing illness, close relatives of CVA patients appeared to have no greater risk of CVA than genetically unrelated individuals.