This study measured the summer mortality rate of breeding waterbirds during a period of three years (1998-2000) in the El Hondo wetland, Spain, an ecosystem of international importance for ornithological diversity. On average, 874 birds died each summer in the study period, but varied from 199 birds in 1998, to 1,690 in 1999. When the observed mortality was expressed as a percentage of the initial breeding populations, the death rates ranged from 1.6% per month in 1998 to 7.7% in 1999. Many of the dead birds gave positive results for the presence of different enterocolic bacteria (30% to 60%) and for the presence of lead shot in their gizzards (30%). These results cannot be however interpreted as percentages causing the ultimate death because many individuals could be only carriers of enterocolic infections and, in addition, many of our positive results for the presence of lead correspond to sub-lethal values. Nevertheless, our results suggest that a significant decrease in the summer mortality rate of waterbirds could be probably achieved by improving the water quality of the ponds in periods of low water levels and by extracting the sources of lead either manually or mechanically.