Tthe population weighted estimates of trade union membership from the HILDA Survey are considerably above the estimates reported each year by the Australian Bureau of Statistics (ABS) from its Employee Earnings, Benefits and Trade Union Membership Survey (which is conducted as a supplement to the August Labour Force Survey). This raises questions about either the nature of the HILDA Survey sample (are union members over-represented) or the way the way union membership is defined and identified in the HILDA Survey instrument.