Lichen simplex is generally regarded as a condition initiated and perpetuated by scratching and emotional tension. It was felt that the scratching might partly be a conditional response to itching and other signals, and that feelings of guilt, anxiety and hostility would be prominent features in these patients. Conditioning experiments designed to establish scratch responses to an itch stimulus (UCS) and a tone (CS) showed that lichen simplex patients conditioned more readily and extinguished more slowly than controls. These finding were more marked when the itch stimulus was applied to affected as compared with normal skin. It was not possible to distinguish differences in the psychological tests between patients and controls. The possible significance of these findings is discussed.