Sensory neurons in the dorsal root ganglia (DRG) and motor neurons in the spinal cord express the 75 kDa low-affinity neurotrophin receptor (p75NTR) during prenatal development. The p75NTR gene knockout mouse provides a unique opportunity to assess the role of p75NTR during this period. Quantitative analysis of the p75NTR knockout mouse revealed a significant developmental loss of sensory neurons. In the cervico-thoracic ganglia approximately 75% of the neurons are lost, while in the lumbar ganglia the loss is approximately 50%. In contrast, motor neurons were not lost in either the cervical or lumbar spinal cord. These data suggest that p75NTR is essential for the prenatal survival of a significant number of sensory, but not motor neurons.