A handy device for testing the thermal sensibility of skin lesions has been developed and field tested in various centres in Africa and India. The instrument performed satisfactorily and its use made testing for thermal sensibility in the field practicable and straightforward. Analysis of the results of testing 260 persons, most of whom exhibited a few lesions that were characteristic of early leprosy, showed that the rate of diagnosis of sensory impairment of such skin lesions, and hence the diagnosis of leprosy, would be about 15-25% more if thermal sensibility testing using this device were added to the other tests of sensibility routinely carried out in the field. Regular use of the device in the field would help to bring more leprosy patients under treatment than at present.