the thermal vasomotor control of digital blood flow has been investigated in 34 subjects. seventeen were suffering from raynaud's phenomenon, ten were controls and seven were suffering from diseases thought to be connected with a malfunction of the sympathetic nervous system. digital blood flow was recorded in one hand while the contralateral hand was subject to a square wave thermal stimulus at various frequencies. using a discrete fourier transformation the flow and stimulus recordings were analysed and their frequency responses compared to find the stimulus frequency that gave the best blood flow response. the results show that raynaud's patients respond better to lower frequencies than do normals, a shift which is indicative of an increase in gain or a decrease in time delay of the thermal vasomotor control loop. the differences between the groups are highly significant (p<0.002) and suggests that thermal response testing may provide a sensitive method of differential diagnosis