“…Factors relevant in causation include ABO blood group (Jick et al, 1969), body weight (Vessey and Doll, 1969), use of oral contraceptives containing oestrogen (McQueen, 1971), and possibly cigarette smoking (Frederiksen and Ravenholt, 1970;Sartwell et al, 1969). Unusual physical effort has been implicated (Feinleib, 1972), but we have found no reference in the literature to occupational factors before the study of Sartwell et al (1969) which found a high proportion of cases of idiopathic venous thromboembolism in a retrospective study to be nurses.…”