Using the projecting method, measurements were made of the size of nuclei of liver cells in liver biopsies taken from ten patients with normal liver histology and normal liver function tests and from two patients with acromegaly. For the statistical analysis a parametric model of the distributions of the nuclear radii, the distributions, was used, making an estimation of the frequencies of di‐, tetra‐ and octaploid nuclei possible. The distribution of the lengths of the nuclear radii was consistent with normal distributions corresponding to the different ploidy classes. Diploid nuclei constituted 84 to 99 per cent of the liver cell nuclei, and the frequency of polyploid nuclei increased with increasing age. Samples of nuclei within the same biopsy revealed significant differences in the mean radius of diploid nuclei and in the frequency of polyploid nuclei. This biological variation must be taken into account in the interpretation of karyometric data. One of the patients with acromegaly had a higher frequency of polyploid nuclei than seen in normal patients, and in both patients with acromegaly the size of diploid nuclei was large compared with normals. The frequency of binuclear nuclei was unrelated to age and frequency of polyploid nuclei, but males were found to have more binuclear liver cells than females.