Immunocytochemical studies on lumbar dorsal root ganglia obtained at routine postmortem 24-36 h after death were carried out, and neuronal cross-sectional areas measured. The subjects were elderly (76-81 years), of both sexes, had died from heart attack or haemorrhage, and had no clinical evidence of clinical neuropathy or of disease known to be associated with neuropathy. The data were consistent between ganglia from the three subjects. There were striking similarities with data from other species. Two populations of cell profiles with overlapping size distributions were distinguished with an anti-neurofilament antibody, neurofilament-rich (45% of cell profiles) with a large mean area and neurofilament-poor with a smaller mean area. Anti-substance P and anti-peripherin antibodies both labelled a population with a small mean area, with extensive co-localization between them. There were also some differences between these human dorsal root ganglia and dorsal root ganglia from some other species. More neuronal profiles were labelled for substance P in humans (44%) than in rat (20%). More neuronal profiles were labelled for SSEA4 (stage specific embryonic antigen 4) in human (40.5%) than in rat dorsal root ganglia (10%), and the SSEA4-positive profiles were relatively smaller in human than in rat. No selective accumulation of lipofusin in profiles of large cells was apparent. This study also shows that quantitative morphometric analysis of immunocytochemically labelled dorsal root ganglion neuronal profiles can be carried out successfully on human sensory ganglia obtained at post-mortem. This is the first demonstration of the two main subgroups of dorsal root ganglia neurones with neurofilament-rich and poor somata in human tissue. The size distributions of neurons with neurofilament, substance P and peripherin are consistent with these neuronal populations having similar functional properties to those described in other species. From the known sensory and fibre loss with aging, it is speculated that the loss of some large diameter neurones with myelinated fibres and low mechanical thresholds, might account for the high percentage of neurones expressing substance P.