An outbreak of shell disease in American lobster (Homarus americanus) over the past several years has generated a great deal of attention and concern regarding its causes and spread into New England coastal waters. An examination of the disease within Massachusetts state waters was conducted from 2000 through 2004. The overall average incidence observed in our data was 3% males and 8% females. Female lobsters were more likely to exhibit disease symptoms than males, and larger lobsters were more likely to possess disease symptoms than smaller lobsters. Disease incidence was highest in May and June, immediately prior to the molting period, and decreased dramatically through the molting season. We detected a north to south latitudinal gradient of increasing shell disease prevalence along the Massachusetts coast. Over the course of our study period, we found relatively constant low levels of shell disease in both the Gulf of Maine and Outer Cape Cod regions. However, a significantly higher level of shell disease was recorded in the Buzzards Bay region, indicating important differences in regional prevalence of disease. Our data suggest that this gradient in shell disease may be related to an interaction between water temperature, sexual maturity, and intermolt duration. There is a significant correlation between disease incidence in Buzzards Bay and a series of warmer than average water temperatures from 1999 to 2003, which suggests that temperature may be a primary factor related to the recent outbreak of epizootic shell disease.