Cutaneous nerve collateral sprouting was studied in 20 adults in whom a forearm cutaneous nerve had been resected from the upper arm, such that any recovery of cutaneous nerve function could not be accounted for by nerve regeneration. Ten patients entered the study immediately following surgery and the remainder at intervals thereafter, permitting a longitudinal study covering a 27-month period. Modality-specific stimuli were used to study light touch, sharp pain, cooling, warming and heat pain sensation. Efferent sympathetic C fibre function was determined by measuring sweating in response to total body heating. Though the patients described considerable subjective reduction in the sensory defect within 2 months, by 10-15 months the objective sensory tests showed encroachment at the margin by only 6 mm (P < 0.05) for light touch, 7 mm (P < 0.01) for sharp pain and 11.5 mm (P < 0.001) for heat pain, with no significant change for warming or cooling. By 24 months, recovery of sweating was evident within the zone of persistent sensory loss, for > or = 3 cm beyond the initial light touch margin (P < 0.005). This finding has important clinical implications as it calls into question the reliance placed on the recovery of sweating as evidence of nerve regeneration.