While it has long been believed that prewar censuses undercounted women's labour force participation, a paucity of independent evidence has constrained the evaluation of potential biases. This article compares participation rates derived from social surveys with censuses for the same towns between 1911 and 1931. The participation rates for women are uniformly lower in the social surveys than in the censuses. An analysis of 35 London boroughs indicates that some of the discrepancy can be explained by differences in the coverage of domestic servants, unemployed, own-account, and part-time workers. Overall there is little evidence that the census undercounted women's work.