The Marfan syndrome is inherited as an autosomal dominant trait, but there is often incomplete penetrance of the gene resulting in a variable phenotype. This variability leads to problems in diagnosis when one is faced with a tall gangly individual with minimal ocular or cardiovascular involvement. In order to confirm the clinical impression of arachnodactyly in patients with the Marfan syndrome, Steinberg¹ in 1966 popularized the thumb sign, which was first described by Parker and Hare² in 1945. This sign is elicited by having the patient make a fist over the clenched thumb. When the result is positive the thumb extends significantly beyond the ulnar margin of the hand (Fig 1). Unfortunately this sign is negative in a sufficiently high proportion of patients with the Marfan syndrome as to reduce its usefulness when used alone (Table). It is also positive in a small proportion of normal children (1.1%