The above overview illustrates the areas of interest in flexion creases in human biology studies in general and in studies of medical disorders in particular. Clearly, flexion creases have a significance of their own rather than only as appendices of the dermatoglyphic analysis and should, therefore, be approached appropriately. In some instances, they may be of more interest than the dermatoglyphics. Our understanding of the creases and their value is, however, as yet incomplete and the gaps in our insight limit the possible interpretations and practical applications of the knowledge gained from studies of various aspects of the flexion creases. Creases provide important clues of the early fetal development and thus may be of practical value in clinical medicine. So far, most of these studies have been carried out in relation to specific disorders, often without a clear reasoning why flexion creases should be altered in the given disorder. The examples of aberrant flexion creases discussed above illustrate the general lack of specificity of crease anomalies in association with a particular syndrome or disorder that does not include malformations or malfunctions of the limbs. Instead, the anomalous flexion creases reflect the altered shape and function of hand and foot, which, in turn, are determined during early fetal development and affected by factors interfering with normal embryogenesis. A more promising approach to studying flexion creases in medical disorders, therefore, is to explore the developmental correlations between the creases and the structure and function of hands and feet. This does not mean that information gained from the case reports of individuals with various congenital defects is of no value. On the contrary, it should be collected systematically to determine the variety of crease aberrations and their frequencies in a given disorder, which would help elucidate the presence and timing of the developmental factors involved in the origin of the defect. The usefulness and limitations of the flexion creases are to a large degree parallel to those of dermatoglyphics. Altered flexion creases are indicative of intrauterine disturbances occurring early in pregnancy. As such, they may be of a predictive value in otherwise apparently normal infants in whom cryptic damage may be manifested later. They may serve to alert the physician to perform careful or more detailed examinations or to reexamine already obtained (and possibly inaccurate) test results. A better understanding of the crease embryology, both normal and abnormal, is a prerequisite for progress in studies of flexion creases.(ABSTRACT TRUNCATED AT 400 WORDS)