We study how an individual's exposure to external information regulates the evaluation of entrepreneurial opportunities and entrepreneurial action. Combining data from interviews, a survey, and a comprehensive web log of an online user community spanning eight years, we find that technical information shaped opportunity evaluation, and social information about user needs drove individuals to entrepreneurial action. Our empirical findings suggest that reducing demand uncertainty is a central factor regulating of entrepreneurial action, an insight that received theories of entrepreneurial action have so far overlooked.