Field studies and aerial photograph interpretation suggest that large sections of Jamaica Bay salt marshes in New York City near John F. Kennedy International Airport are deteriorating rapidly. The relatively recent salt marsh losses may be caused by a variety of factors, potentially interacting synergistically. Possible factors include reduced sediment input, dredging for navigation channels, boat traffic, and regional sea-level rise. Field work included aboveground biomass measurements of Spartina alterniflora, mapping plant community distribution, and documenting biogeomorphological indicators of marsh loss. Current productivity (standing crop biomass), which ranged from approximately 700 to 1500 g m⁻², was typical of healthy marshes in this region, in spite of other indicators of marsh degradation. Historical aerial photographs of several islands showed that sampled marshes have diminished in size by ∼12% since 1959. Overall island low marsh vegetation losses since 1974 averaged 38%, with smaller islands losing up to 78% of their vegetation cover. Ground observations indicate that major mechanisms of marsh loss include increased ponding within marsh interiors, slumping along marsh edges, and widening of tidal inlets. Projections of future sea-level rise, using outputs from several global climate models and data from local tide gauges, in conjunction with a range of plausible accretion rates, suggest that under current stresses, Jamaica Bay salt marshes are unlikely to keep pace with accelerated rates of sea-level rise in the future.