The American lobster, Homarus americanus H. Milne-Edwards, 1837, supports the most economically important shellfish fishery worldwide, with the state of Maine contributing the majority of US landings. Incidence of epizootic shell disease (ESD) is increasing along the Maine coast, highlighting the need to increase our understanding of factors affecting its etiology. Molting lessens the severity or eliminates the disease, and lobsters in the wild with shell disease have been shown to have increased concentrations of hemolymph ecdysteroids, key steroid hormone regulators of molting. Here, we examine the effects of cold, moderate, and warm temperature regimes mimicking annual temperature cycles from regions along the New England coast on hemolymph ecdysterone (20E) concentrations. At baseline sampling from lobsters collected in the field, it was determined that 20E was significantly elevated in lobsters with severe shell disease, but not in those with low to moderate levels of the disease. Hemolymph 20E concentrations correlated with relative molt stage, and were significantly higher in summer than winter for all temperature regime treatments. Lobsters with shell disease were more likely to molt, and more likely to die, than lobsters with no signs of the disease, regardless of temperature treatment. In the winter sampling period, 20E was highest in lobsters from the warm temperature regime, but not in summer. Therefore, these data suggest that temperature regime does impact hemolymph 20E concentrations in both diseased and non–shell diseased lobsters, and is consistent with the hypothesis that elevated 20E may be a mechanism to mitigate the effects of shell disease.