As measurement of absolute glomerular size is difficult we developed a method of assessing glomerular size that was simple and practical and could be used to compare the kidneys in different groups of patients. Using a semi-automatic image analyser, the cross-sectional area of 100 randomly-selected glomeruli, outlined by Bowman's capsule, was measured on sections of kidneys taken at necropsy. The mean of the logarithms of the largest 25 areas was calculated. The method was applied to compare control kidneys (53) with the kidneys in acromegalics (20), in patients with one kidney (10) and in patients with asymmetrical kidneys (12). Kidneys were heavier in the three test groups than in controls. Glomerular sizes were similar in controls and in acromegalics but were larger in single and disparate kidneys. There was a relationship between glomerular size and kidney weight within the control group and across the four groups taken together. This only partly accounted for the observed differences in glomerular size between the groups. Histological comparison of the acromegalic and single kidneys showed more global glomerulosclerosis in single kidneys and also segmental lesions, mainly at the glomerular hilum, only in the single kidneys. These findings show that renal enlargement occurs in acromegaly and in single and disparate kidneys but is accompanied by markedly different glomerular features. This implies different mechanisms for the renal enlargement. The method of assessing glomerular size is useful in the study of these and other conditions affecting the kidney.