The risk of an abnormal palmar flexion crease was 36.0% in the discordant affected twins (N=9), 32.4% in the concordant affected twins (N=11), and 16.0% in the discordant nonaffected twins (N=4). The risk of ridge count dissociation was 9.7% in the discordant affected twins (N=3), 10.5% in the concordant affected twins (N=4), and 6.5% in the discordant nonaffected twins (N=2). The risk of either ridge dissociation or abnormal palmar flexion creases was higher in the combined group of affected concordant and affected discordant twins (37.7%, 23 of 61) than in the group of discordant nonaffected twins (20.0%, five of 25; odds ratio=2.4, 95% CI=0.8–7.1; χ2=2.53, df=1, one-sided p=0.06, one-sided exact p=0.09). Of the 31 discordant pairs, nine were discordant for abnormal palmar flexion creases. In seven of these nine pairs, the abnormality occurred in the affected twin (odds ratio=3.5, 95% CI=0.7–16.8, χ2=2.78, df=1, one-sided p=0.05, one-sided exact p=0.09). Similarly, of the three pairs discordant for ridge count dissociation, the abnormality occurred in the affected co-twin in two pairs (odds ratio=2.0, 95% CI=0.2–22.0, χ2=0.33, df=1, one-sided p=0.28, one-sided exact p=0.50). For the discordant pairs the odds ratio associated with the presence of either abnormal palmar flexion creases or ridge count dissociation in the affected twin was 3.0 (95% CI=0.8–11.1, χ2=3.00, df=1, one-sided p=0.04, one-sided exact p=0.07). Of the nine concordant pairs in which at least one twin had either ridge count dissociation or abnormal palmar flexion creases, only three pairs were also concordant for the dermatoglyphic abnormality.