This article contributes to the occupational choice literature pertaining to entrepreneurship by applying the Theory of Planned Behaviour (TPB) to predict entrepreneurial behaviour. Originating from social psychology, the TPB posits that intention, a function of behavioural beliefs, is a significant predictor of subsequent behaviour. In spite of an established stream of scholarship explaining the formation of entrepreneurial intentions, empirical research has not yet employed longitudinal data to examine whether the intention to start a business measured at one point of time translates into subsequent entrepreneurial behaviour. This article provides a full test of the TPB in the prediction of business start-up intentions and subsequent behaviour based on two-wave survey data (2006 and 2009) from the working-age population in Finland. The econometric results support the predictions outlined in the TPB: attitude, perceived behavioural control and subjective norms are significant predictors of entrepreneurial intention; and intention and perceived behavioural control are significant predictors of subsequent behaviour. This research thus provides support to the application of the TPB and the concept of behavioural intention to understand the emergence of complex economic behaviour such as entrepreneurship prior to the onset of any observable action.