["Neuropsychiatric symptoms related to aciclovir or valaciclovir treatment have been a problem since aciclovir was introduced in the early 1980s. We have previously found that subjects with aciclovir-related neuropsychiatric symptoms have increased serum concentrations of aciclovir's main metabolite, 9-carboxymethoxymethylguanine (CMMG). The aim of this study was to investigate whether CMMG was present in the CSF of aciclovir- or valaciclovir-treated subjects with or without neuropsychiatric side effects that appeared during therapy.", 'We investigated retrospectively CSF collected from 21 aciclovir- or valaciclovir-treated subjects. Of these, 9 were subjects with neuropsychiatric signs and symptoms and 12 were asymptomatic subjects, including 10 subjects from a valaciclovir multiple sclerosis trial and 2 subjects with recurrent herpes encephalitis.', 'CMMG could only be detected in the CSF of subjects with neuropsychiatric symptoms and signs (median CMMG concentration 1.0 micromol/L, range 0.6-7.0). The concentration of CMMG was below the limit of quantification (<0.5 micromol/L) in asymptomatic subjects (P < 0.001). All patients with neuropsychiatric signs and symptoms, except one, had acute renal function impairment or chronic renal failure.'] Is the aciclovir metabolite CMMG detectable in the CSF of subjects with neuropsychiatric symptoms during aciclovir and valaciclovir treatment?