The presence of anticardiolipin antibodies (aCL) has been associated with arterial and venous thrombotic events in connective tissue diseases. Previous investigations have suggested an increased incidence of aCL in the elderly population. We have studied the prevalence of aCL in large groups of 300 healthy elderly (mean age 70) and 543 younger subjects. aCL were determined by ELISA for the presence of IgG and IgM antibodies and was detected in 37 individuals (12%). This compared with an overall prevalence in a younger population of 2%. In addition, aCL was detected in 23% of elderly individuals who were also positive test for antinuclear antibodies (ANA). There was, however, no correlation with the presence of rheumatoid factor or lymphocytotoxic antibodies in this elderly group. Therefore, aCL have increased prevalence in an elderly population, and were associated with a positive test for ANA.