Spinal anaesthesia was compared in 40 obese patients (increased body mass index (BMI] and 40 patients with normal BMI when 3 ml of plain 0.5% bupivacaine was injected at either the L3-4 or L4-5 interspace. More extensive cephalad spread of sensory block was achieved in patients with increased BMI compared with patients with normal BMI after injection at both L3-4 and L4-5 (P less than 0.05). The use of the L3-4 interspace instead of L4-5 resulted in a higher mean spread of block in patients with both increased (T4 vs T8) or normal (T9 vs T11) BMI (P less than 0.05). The interindividual variability of the blocks was relatively small in patients with normal BMI injected at L4-5, compared with the three other groups (normal BMI injected at L3-4, increased BMI injected at L3-4 and L4-5). Good anaesthesia was produced in all patients for orthopaedic surgery of the lower extremity. In an obese patient it is recommended that plain bupivacaine be administered at L4-5 instead of L3-4 when extensive spread of the block is to be avoided.