Brief heat stimuli, elicited by a CO2 laser (10.6 microns wave length), activate the most superficial cutaneous nerve terminals of the thin myelinated A delta and unmyelinated C fibres which mediate heat and pain sensations. This paper investigates late cerebral potentials (SEPs) in response to laser pulses in comparison with those to conventional electrical stimulation in 18 patients with a dissociated sensory deficit (intact mechano-sensibility and disturbed temperature and pain sensation). Patients were stimulated in the most disturbed limb (affected area) and in a corresponding control area. In all 18 patients the SEPs elicited by laser stimuli were able to identify the body site with heaviest disturbances in pain and thermo-sensibility: the SEPs from the affected area were reduced or delayed, compared to the control area. In contrast, no alterations in SEPs could be observed after conventional electrical nerve stimulation, in agreement with the normal mechano-sensibility. However, the degree of SEP modulation in response to cutaneous heat stimuli did not correspond to the severity of the subjectively reported sensory deficit. Highest correlations between sensory deficits and abnormal SEPs were found in all those patients in whom computer tomography or MR imaging documented a localized destructive process in the CNS. All patients with the smallest SEP modulations despite a considerable sensory deficit had an inflammatory aetiology. Preliminary criteria to define a laser-evoked SEP as pathological are discussed.