Existing long-term monitoring data and studies initiated in response to the 1999 lobster die-off in Long Island Sound were examined to determine long-term trends that might clarify causes of the die-off. Data examined included a 28-y time series of commercial lobster-trap catch (harvest and discard) sea-sampling, a 20-y time series of research trawl survey indices, a 13-y time series of bottom water temperature, 3 y of mark-recapture data and 1 y of a research trap survey. Movement information was gathered from the recapture of 2,309 lobsters at large within the Sound for more than 30 days (average days at large = 177). Only 9% of the recaptured lobsters traveled more than 10 km from their release point and 1.3% traveled more than 20 km. Based on the recaptures reported over 3 y, it appears that most lobsters remain resident in the Sound and do not travel extensive distances. The eastern portion of Long Island Sound contained the majority of lobsters that moved greater distances. Abundance of lobsters by size class taken in the trawl survey showed a rise in overall abundance during the 1990s due to a substantial increase in the abundance of pre-recruit and recruit size lobsters (<82 mm CL). Following the die-off, abundance of all size classes declined sharply. The increase and decrease in abundance of the sublegal size classes were negatively correlated with mean summer bottom water temperature. Following the die-off, the percentage of females that were egg-bearing in catches from the western Narrows, the area hardest hit by the die-off, was significantly lower compared with the rest of the Sound. It appears that egg-bearing females were most vulnerable to mortality factors causing the die-off, and/or these factors may have limited their ability to carry eggs. Observed mortality in the commercial catch in the western Sound also correlated with mean summer bottom water temperature over 8 y bridging the die-off (1996–2003). Multiple factors probably played a role in the higher-than-normal mortality recorded in 1999, however, the long-term monitoring data examined here implicate increased bottom water temperatures as a significant contributing factor.