Several studies have found effects of orthographically related masked nonword primes on lexical decisions to target words. These effects have been explained by the neighborhood characteristics of the target word (Forster, 1987), but the neighborhood characteristics of the prime in combination with the target are also found to be important (Hinton, Liversedge, & Underwood, 1998). In this study, we present a new account of masked form-priming effects based on the shared neighborhood of prime and target. Shared neighbors are words that are activated by both prime and target. According to the interactive activation model (McClelland & Rumelhart, 1981), shared neighborhood determines the size of priming effects. This prediction was tested and confirmed in a masked priming experiment that manipulated the shared neighborhood density of complete primes.