An association between arachnoid cysts of the middle fossa (ACMF) and complicating subdural haematoma (SDH) has been previously noted. More recently it has been hypothesized that ACMF may predispose to SDH formation. The Plymouth Neurosurgical Unit has treated twenty patients with ACMF between 1976-1985, seven of these being complicated by SDH. There was an age range of 11-56 years in those with SDH. Six of the seven patients with ACMF and SDH gave no significant trauma history. Four of these were males aged 11 to 20 years. The presenting histories, clinical findings and subsequent management of these patients were compared with the age-matched males with SDH alone (twelve patients). In the SDH alone group 100% suffered major skull trauma and 80% had demonstrable skull fractures. In addition the patients with ACMF were compared with patients presenting with other supratentorial arachnoid cysts in Plymouth. Only ACMF were associated with the development of SDH in our study. Three patients demonstrated total masking of the ACMF by isodense intracystic haematoma on computed tomography. In two of these patients the presence of an ACMF was suspected due to plain radiographic and CT enlargement of the middle fossa. It is advocated that with special reference to young males, in the absence of major skull trauma an ACMF should be suspected as a predisposing factor to SDH. Postoperative CT scans for at least one year are advisable in young patients with SDH as demonstration of the presence of an ACMF and SDH affects future management.