Cutaneous reactivity to challenge with dinitrochlorobenzene (DNCB) and diphenylcyclopropenone (DCP) was studied in 16 volunteers following hypnotic suggestions to increase and decrease response during sensitization. The immunoreactivity to DNCB and DCP was modulated by direct suggestions and guided imagery under hypnosis. The volunteers were highly susceptible subjects selected by means of the Harvard Group Scale of Hypnotic Susceptibility, Form A. Measurement of skin reactions to the challenge 1 month after sensitization was performed double blindly. Results showed a significant (P < 0.01) difference in visually scored reactions to DCP and DNCB between the group instructed to increase reaction to DCP and decrease reaction to DNCB and the group given the opposite instructions. A non-significant difference (P = 0.055) in skin thickness measured by ultrasound was found between the two groups. The study supports previous reports of experimental modulation of immunoreactivity and indicates that the specific immunological processes involved in the development of allergic reactions may be susceptible to psychological factors.