We investigate the merit of deriving an estimate of the basic reproduction number R0 early in an outbreak of an (emerging) infection from estimates of the incidence and generation interval only. We compare such estimates of R0 with estimates incorporating additional model assumptions, and determine the circumstances under which the different estimates are consistent. We show that one has to be careful when using observed exponential growth rates to derive an estimate of R0, and we quantify the discrepancies that arise.