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Background and purpose The severity of Wilson's disease ( WD) is linked to free copper accumulating in the liver and brain. Exchangeable copper (Cu EXC) is a new technique to determine plasmatic copper and is useful in the diagnosis of WD. It is hypothesized that it may also enable a good evaluation of extra-hepatic involvement and its severity. Methods Forty-eight newly diagnosed WD patients were prospectively evaluated using hepatic, neurological, ophthalmological and brain magnetic resonance imaging ( MRI) scores. Three phenotypic presentations were distinguished: pre-symptomatic, hepatic and extra-hepatic. Cu EXC was determined in addition to standard copper assays before decoppering therapy. Correlations between biological parameters and the different scores were determined and compared in the hepatic and extra-hepatic groups. Results Extra-hepatic patients had significantly higher Cu EXC values than those with the hepatic form ( P < 0.0001). The overall ability of Cu EXC to separate the two forms was satisfactory, with an area under the curve of 0.883 (95% confidence interval 0.771-0.996) and an optimal threshold for extra-hepatic diagnosis of 2.08 μmol/l (sensitivity 85.7%; specificity 94.1%). In extra-hepatic patients, Cu EXC was the only biological marker to be positively correlated with the Unified Wilson Disease Rating Score ( r = 0.45, P = 0.016), the Kayser−Fleischer ring score ( r = 0.46, P = 0.014) and the brain MRI score ( r = 0.38, P = 0.048), but it was not correlated with the hepatic score. Conclusions Exchangeable copper determination is useful when diagnosing WD as a value >2.08 μmol/l is indicative of the severity of the extra-hepatic involvement. In the case of purely hepatic presentation, atypical or mild neurological signs, it should encourage physicians to search for lesions in the brain and eyes. [ABSTRACT FROM AUTHOR]