Corticobasal degeneration (CBD) is a rare syndrome characterised by an asymmetrical rigidity with localised cortical signs, particularly apraxia. Using positron emission tomography, abnormal patterns of cortical metabolism have recently been shown. We have studied patterns of regional cerebral blood flow (rCBF) using single photon emission tomography, with the tracer 99Technetium hexamethylpropylenamine (HMPAO), in subjects with CBD. In subjects with CBD, compared with 12 age-matched normal controls, in the clinically more affected hemisphere a characteristic pattern was found with significant reductions in HMPAO uptake in the posterior frontal cortex (by 11.5%), and in the superior, inferior, anterior, and posterior parietal cortex (by 12.2, 12.9, 12.9, and 9.7, respectively). Reduced uptake was also found in the caudate (9.3%), putamen (9.7%), and thalamus (8.6%). In contrast, HMPAO uptake in the temporal and occipital cortex was normal. In comparison with 12 Parkinson's disease (PD) controls, significant reduced uptake was seen in the thalamus (9.0%), posterior frontal (8.9%), and inferior (9.9%), and anterior parietal (9.5%) cortex. A similar pattern of impaired uptake was seen in the clinically less/unaffected cerebral hemisphere in patients with CBD compared with normal controls, with a significant reduction in HMPAO uptake in the thalamus (6.8%), superior parietal (9.6%) and anterior parietal (7.6%), and posterior parietal (7.3%) cortex. This implies that the disease process is bilateral even in those cases with clinically unilateral disease. This widely available technique may be useful in the early diagnosis of CBD and in differentiation from other extrapyramidal disorders.