Patients admitted to casualty departments with acutely disturbed behaviour present a major diagnostic challenge. The differential diagnosis includes both primary psychiatric illness and a wide range of organic acute brain syndromes, including substance abuse. Certain toxic syndromes indicating a specific substance may become familiar to medieal staff owing to frequent local abuse. An infusion of B sauveolens, regularly used in the Brisbane region, causes a central anticholinergic syndrome with wildly disturbed behaviour, visual hallucinations, and dilated pupils.2 There was an initial suspicion that this poison had been ingested by our two patients. Many infections of the central nervous system may have prominent psychiatric manifestations, leading to initial misdiagnosis, psychopharmacological intervention, and delay in starting appropriate treatment.`7 Both the young adult patients described above had pyogenic meningitis. Both developed wildly disturbed behaviour of sudden onset with clouding of consciousness but without other, more typical, features of central nervous infection such as fever or neck rigidity. In each case the primary diagnosis was substance abuse, and initial treatment was given accordingly. An important clue, however, to the true diagnosis in both was the finding of leucocytosis. Diagnostic lumbar puncture and administration of antibiotics were delayed while cerebral computed tomography was performed. To facilitate this investigation both patients were sedated and ventilated, thus preventing further neurological assessment. There is disagreement among neurologists as to whether lumbar puncture should proceed without tomography in these circumstances. However, if an immediate scan is available then most would advocate its initial use to minimise the risk of coning. To avoid potentially disastrous delays in treatment clinicians should be aware that bacterial meningitis can present quite atypically, with the sudden onset of severe behavioural disturbance closely mimicking substance abuse. Previously well patients presenting with acute behavioural disturbance and leucocytosis without clear evidence of substance abuse require urgent lumbar puncture and appropriate antibiotics. If immediate cerebral computed tomography is available it may be used before lumbar puncture, although the patient should be given broad spectrum antibiotics first. This is particularly important should short term ventilation be required, and antibiotic therapy should never be withheld while the results of investigations are awaited.8