IT is now widely accepted that many cases of severe and persistent sciatica are due to compression extrathecally of either the fifth lumbar nerve root by a protrusion from the fourth lumbar intervertebral disc, or the first sacral nerve root by a protrusion from the fifth lumbar disc(Bradford and Spurling, 1941; Falconer, 1944; etc.). It is also known that, in such cases, areas of sensory impairment, particularly of pain and tactile sensibility, often. appear in the affected leg. These areas, however, are difficult to chart, for the sensory loss is only partial, and it is generally assumed that they are confined to the foot and leg below the knee. In fact some authorities (Love and Walsh, 1938; O'Connell, 1943) presume that, if sensory changes extend above the knee, more than one nerve root must be involved, and that sacral roots within the theca are affected as well as the extrathecal nerve root at the level of the disc protrusion. Few appear to realize that, if sensation is tested carefully, a slight -disturbance can usually be detected in the thigh and bpttock as well as below the knee, even though only a single nerve root is involved, and that by careful study of these areas of sensory disturbance it is often possible to identify the exact nerve root concerned. Until a late stage in our own experience we did not realise that compression of either of these two nerve roots commonly produces patterns of sensory impairment which extend well above the knee. We then learnt that, if sensation is tested in a simple but standardized manner, it is frequently possible to outline a pattern of sensory change (verifiable by other examiners) which is characteristic of the particular nerve root, and hence is of diagnostic value. Since then further observations have been made which indicate that these areas are in reality dennatomes, and that the classical descriptions of the dermatomes by Head (1893), D6jerine (1914), and Foerster (1933), and even the more recent description by Keegan (1943) are faulty. Our observations instead support and expand ideas on these particular dermatomal areas put forward by Thorburn in 1893. We have also observed that areas of hyperalgesia may occur within the dermatomes, and further that, when pain is severe, sensory impairment may become more extensive than can be explained on a segnental basis alone.