We investigated the winter denning patterns of Porcupines (Erethizon dorsatum) in Massachusetts during two winters (1991-1992 and 1992-1993) in two study areas that differed in both availability of den sites and types of dens to assess whether frequency of den sharing was related to den density, den distribution, weather, or mating season. In the Central area, where there were fewer available den sites and fewer dens used per individual, Porcupines which denned together did so relatively often (64-78% of locations in dens). However, in the East area, where den site availability was higher, Porcupines denned together less frequently (9-37%). Fewer Porcupines denned with another Porcupine at least once in the Central area (45 vs. 70%), but dens were more dispersed there and five porcupines rested in trees and never used a den in winter. In addition, more severe winter weather reduced sharing. Male-female pairing was not limited to mating season and mating activity did not help to explain late winter den sharing.