The way firms lengthen or shorten their product line with respect to rivals is regarded as one of the possible strategies firms can pursue to respond to competition. This article builds and tests hypotheses to study the effect of different levels of competitive intensity on product line length. The empirical analysis of data on 3,527 handset models introduced by 66 mobile phone vendors from 1994 to 2010 shows a consistent inverse U‐shaped relationship between competitive intensity and the firm's product line length. In this way, we pinpoint an interesting link between the product line extension literature and the competitive dynamics and competitive intensity perspectives. Copyright © 2013 John Wiley & Sons, Ltd