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Since its recognition, progressive multifocal leucoencephalopathy (PML), a highly morbid and often fatal disease caused by the John Cunningham virus (JCV), has left neurology and infectious disease clinicians with a sense of therapeutic nihilism. This reputation is well earned. For a long time, the clinical task of diagnosing PML, withdrawing immunosuppression and restoring what immune system function can be restored, often resulted in a disappointing outcome. Recent therapeutic developments, however, bring great hope in confronting this disease and, with it, the need for an early and accurate diagnosis. The accompanying review by Sawicka et al is both timely and important.1 It highlights the evolving epidemiology of PML, reminds us to consider the disease in seemingly immunocompetent patients (who may have sarcoidosis, systemic lupus erythematosus or an underlying immunodeficiency) and emphasises the importance of characterising the PML ‘phenotype’. Moreover, the proposed diagnostic algorithm emphasises characteristic neuroimaging features and the use of repeat or ultrasensitive JCV PCR testing in highly suspicious (but initially PCR negative) cases as additional tools to support a probable or definite diagnosis of PML. Accurately diagnosing PML is now the necessary first step in determining which patients may be eligible for current therapeutics under investigation.