The recently developed Stark-adapted quantum defect orbital (SQDO) formalism has been employed to calculate field ionisation thresholds for s, p and d states of sodium in the range n = 15-20. The passage from lower to higher fields has been considered to be completely adiabatic. The presently reported values appear to be in very good agreement with the available experimental data. (C) 2004 Elsevier B.V. All rights reserved.