The spleen is an important organ of vertebrates. Splenic mass can change in response to a variety of factors. We tested whether splenic mass of masked shrews, Sorex cinereus, was related to sex, age, time of the year, or intensity of bladder nematode (Liniscus [=Capillaria] maseri) infection, after controlling for host body mass. For females, body mass was a strong predictor of splenic mass. For males, splenic masses were greater later in the year and in more heavily infected males. The latter appeared to represent a threshold response wherein only the most heavily infected individuals had enlarged spleens.