THE existence of a posterolateral thalamic relay nucleus for pain and temperature sensation was postulated in 1911, on the basis of the stroke-induced analgesia and thermanaesthesia found paradoxically in patients with thalamic pain syndrome(1). Pain or temperature sensations can be evoked in humans by electrical stimulation in a vaguely defined region of the posterolateral thalamus(2,3). Here we use anterograde tracing and single unit recordings to demonstrate that there is a distinct nucleus in the posterior thalamus of the macaque monkey that receives a dense, topographic input from spinothalamic lamina I neurons and in which almost all neurons are nociceptive- or thermoreceptive-specific. Immunohistochemical staining showed that this nucleus is defined by a dense calbindin-positive fibre plexus in the macaque, so we applied the same staining method to sections of human thalamus. We found a nearly identical fibre plexus localized within a distinct nucleus that is cytoarchitectonically homologous to the lamina I relay nucleus in the macaque thalamus. The stereotaxic coordinates of this nucleus and its location relative to the main somatosensory representation fit clinical descriptions of the pain-producing region in humans. We conclude that this is a specific thalamic nucleus for pain and temperature sensation in both monkey and human.