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There are several reasons why end-to-side nerve coaptation has not been widely adopted clinically. Among these are the putative damage inflicted on the donor nerve and the variable quality of the regeneration in the recipient nerve. So far experiments on end-to-side nerve repair have been short term and mostly carried out on rats. This long-term study of end-to-side nerve repair of ulnar to median and median to ulnar nerve was performed using adult nonhuman primates. Eleven nerve repairs were studied at different time points. Eighteen, 22, 33 and 57 months after surgery a qualitative and quantitative analysis of the donor nerve and regenerating nerve revealed variable levels of percentage axonal regeneration compared with matched controls (1.4%-136%). Morphological evidence of donor nerve damage was identified distal to the coaptation site in four of the 11 cases, and in these cases the best axonal regeneration in the corresponding recipient nerves was observed. This donor nerve damage could neither be demonstrated in terms of a decrease in axon counts distal to the coaptation nor as donor target organ denervation. Recipient target organ regeneration like the axonal regeneration varied, with evidence of motor regeneration in eight out of 11 cases and sensory regeneration, as measured by percentage innervation density compared with matched controls, varied from 12.5% to 49%. Results from the present study demonstrate that the end-to-side coaptation technique in the nonhuman primate does not give predictable results. In general the motor recovery appeared better than the sensory and in those cases where donor nerve damage was observed there was better motor and sensory regeneration overall than in the remaining cases.