Utilizing the Golgi technique, the present study provides a structural analysis of primate marginal (lamina I) neurons in the lumbosacral spinal cord. Marginal neurons are classified on the basis of major structural differences in dendritic conformation, distribution, and specialization. Cell size and shape alone were not found to be reliable criteria. Marginal cells can be divided into four major groups. Group I (Aspiny Neurons with Thick, Blunt Dendrites) consists of neurons with relatively thick dendrites which have an abrupt, blunt termination and few spines. This heterogeneous group includes large, medium, and small neurons of various shapes. Group II cells (Large to Medium Spiny Neurons) can be subdivided into two distinct groups: Group IIA neurons, which have longitudinal spiny dendritic arbors, and Group IIB cells, which have a moderately spiny, fan-shaped dendritic arbor which spreads across the lateral portion of the dorsal marginal zone. Both Groups A and B exhibit several types of spines. Group III (Aspiny Neurons with Thin, Tapering Dendrites) consists of small to medium size neurons which can be further divided into two groups: Group IIIA, which is characterized by oval- to fusiform-shaped neurons with tortuous, fine, tapered dendrites which ramify in the dorsolateral fasciculus and the lateral funiculus, and Group IIIB, which is composed of fusiform-pyramidal-and polygonal-shaped neurons with fine, tapering dendrites confined to lamina I. Group IV (Small Spiny Neurons) are characterized by a small fusiform- to pyramidal-shaped cell body and delicate longitudinal dendrites with small, short-necked pedunculated spines. This group is subdivided into Group IVA cells, which are found within lamina I proper and Group IVB cells, which are located in the dorsolateral fasciculus and have unmyelinated axons. The present study demonstrates considerably more structural diversity within the marginal zone than has been previously reported, and offers sufficient variation to correlate with functional differences described from laminal I neurons.