This paper statistically tests the option theory of irreversible investment under uncertainty. Using contingent claims valuation, we derive the value of options to invest in capacity, where the projects are endogenous to the economic circumstances prevailing at the investment date. We then test whether capacity investment decisions made by Canadian copper mines are compatible with the trigger price implied by the theory. The results speak strongly in favor of option theory as a theory of real investment. Our model explains both investment size and timing satisfactorily, from a statistical, and from an economic, point of view, and numerical simulations with a mean-reverting process suggest that the results do not depend crucially on the price being assumed to follow a geometric Brownian motion.