The total cortical and striatal neurone and glial numbers were estimated in five cases of Huntington's disease (three males, two females) and five ageand sex-matched control cases. Serial 500-l-lm-thick gallocyanin-stained frontal sections through the left hemisphere were analysed using Cavalieri's principle for volume and the optical disector for cell density estimations. The average cortical neurone number of five controls (mean age 53±13 years, range 36-72 years) was 5.97x 10 9 ±320x 10 6 , the average number of small striatal neurones was 82 X 10 6 ± 15.8 X 10 6 • The left striatum (caudatum, putamen, and accumbens) contained a mean of 273 X 10 6 ±53 X 10 6 glial cells (oligodendrocytes, astrocytes and unc1assifiable glial profiles). The mean cortical neurone number in Huntington's disease patients (mean age 49±14 years, range 36-75 years) was diminished by about 33 % to 3.99x10 9 ±218x10 6 nerve cells (P : : ; : : : : 0.012, MannWhitney V-test). The mean number of small striatal neurones decreased tremendously to 9.72 X 10 6 ±