The incidence of postdural puncture headache (PDPH) following spinal anaesthesia with a 0.33 mm (29-gauge) and two types of 0.7 mm (22-gauge) Whitacre needles was investigated in 400 patients less than 40 years old. The incidence of PDPH was 2% in the 0.33 mm group, and 3.5% in the 0.7 mm group. Headache of other origin was seen in 12 patients (6%) in the 0.33 mm and in five patients (2.5%) in the 0.7 mm group. These differences were not significant. The failure rate was significantly higher in the 0.33 mm group (8.5% vs 2%) than in the 0.7 mm group (P less than 0.05). It is concluded that the 0.33 mm needle is associated with a low incidence of PDPH in young patients, but has a significantly higher failure rate than the Whitacre 0.7 mm needle, which is also a suitable choice in this age-group because of its ease of handling and the low incidence of PDPH.