Using intermediate weighted magnetic resonance imaging (MRI) and a systematic sampling stereological method in 39 normal volunteers aged 24-79 years old, we demonstrated a marked age-associated decline in caudate nuclei volume (r = -0.69, p less than 0.0001). The mean absolute volume of the caudate nuclei in this study (9.4 cm3) was almost identical to that reported in a previous autopsy study and further confirms the validity of this stereological technique for use with MR images. This technique will provide a method for measuring the caudate and other nuclei in vivo, from brain images and, as such, a research tool to correlate age-associated changes in cognitive, sensory and motor function with caudate nucleus volume and other brain regions.