This is the first large series, comprising 50 patients who suffered a total of 164 episodes, of pseudomigraine with temporary neurological symptoms and lymphocytic pleocytosis (PMP syndrome). Onset of PMP was between the ages of 14 and 39 years and was most frequent in males (68%). Eight males (24%) and five females (31%) had a personal history of migraine. One-quarter had had a viral-like illness up to 3 weeks prior to the onset of the syndrome. The clinical picture consisted of one to 12 episodes of changing variable neurological deficits accompanied by moderate-to-severe headache and occasionally fever. The headaches were described as predominantly throbbing and bilateral with variable duration (mean, 19 h). The mean duration of the transient neurological deficits was 5 h. Sensory symptoms were most common (78% of episodes), followed by aphasic (66%) and motor (56%) symptoms. Visual symptoms appeared in only 12% of episodes. The most frequent combinations were motor aphasia plus sensory and motor right hemibody symptoms (19% of episodes), motor aphasia plus right sensory symptoms (10%) and isolated right (9%) or left (9%) sensory symptoms. All patients were asymptomatic between episodes and following the symptomatic period (maximum duration 49 days). Lymphocytic pleocytosis ranged from 10 to 760 lymphocytic cells/mm3 CSF (mean, 199). In CSF, protein was increased in 96% of patients, IgG was normal in 80% of cases and oligoclonal bands were not found. Adensoine deaminase values were slightly above normal in two out of 16 patients tested. Extensive microbiological determinations, including viral HIV and borrelia serologies, were negative. Brain CT and MRI were always within normal limits, while EEG frequently showed focal slowing. Conventional cranial angiography was performed on 12 patients. In only one were there abnormalities suggestive of localized vascular inflammation, coincident with the focal neurological symptoms. Two patients developed PMP symptoms immediately after angiography. SPECT, performed on only three patients in the symptomatic period, revealed focal areas of decreased uptake consistent with the clinical symptoms. PMP aetiology remains a mystery; chronic arachnoiditis, viral meningoencephalitis or migraine are not plausible aetiological explanations. Because a number of patients had had a prodromic viral-like illness, we hypothesize here that such a viral infection could activate the immune system, thereby producing antibodies that would induce an aseptic inflammation of the leptomeningeal vasculature, possibly accounting for this clinical picture.