Five out of nine adults (55%) with lymphoblastic disease developed severe avascular necrosis of bone (AVN) when treated with a Berlin-Frankfurt-Munster (BFM) ALL protocol similar to the current joint MRC-ECOG ALL trial (UKALL XII). The principal purpose of these intensified regimens is to improve long-term disease-free survival without necessarily increasing toxicity and secondary morbidity. The presentation of all five was non-specific bone pain occurring after the re-intensification block of chemotherapy containing high doses of dexamethasone. Three types of diagnostic imaging were performed and magnetic resonance imaging (MRI) proved superior in demonstrating AVN and showed it at an earlier stage than plain radiographs or isotopic scans. We believe that the dose of corticosteroids was the major factor in the development of AVN. The five men in our series all remain in first remission with a median disease-free survival of 3.5 years (range 2-8 years) but with varying degrees of disability due to AVN. Clinicians involved in UKALL XII and similar trials should be aware of this debilitating and potentially crippling complication when using high-dose steroid-containing regimens, perform MRI scan early and modify treatment if necessary.