Clinical and genetic characteristics of histologically defined fissured tongue were examined in a familial study. Fifteen probands with fissured tongue and four probands with geographic tongue were selected from earlier studies. In addition, 12 probands with tongue fissuring, but without changes of papillary structure, were included. The total sample consisted of 31 families; the number of family members examined was 185 (93 men, 92 women), and the mean age of the subjects was 20 yr (range 1-78). Diagnosis of tongue form was emphasized, and this study describes an in vivo method of stereomicroscopy for examining the dorsum of the tongue. According to genetic analysis, fissured tongue with smooth-surfaced papillae was transmitted as a dominant characteristic with incomplete penetrance and was preceded by geographic tongue. The severity of fissured tongue changed with increasing age. Tongue fissuring with normal-appearing filiform papillae was not familial and was not associated with geographic tongue. Fissuring with normal papillary structure should be considered as variations of normal anatomy, whereas fissured tongue and geographic tongue are a clinical and etiological disease entity.