During the period from 1990 to 1997, annual rainfall accumulation averaged 87% of normal at the 12 stations with the longest period of record in Puerto Rico, a Caribbean island with a 1999 population of 3.8 million. Streamflow in rivers supplying the La Plata and Loi??za reservoirs, the principal water supply of the San Juan metropolitan area, was at or below the 10th flow percentile for 27% to 50% of the time between December 1993 and May 1996. Diminished reservoir levels in 1994 and 1995 affected more than 1 million people in the San Juan metropolitan area. Water rationing was implemented during this period and significant agricultural losses, valued at $165 million, were recorded in 1994. The public endured a year of mandatory water rationing in which sections of the city had their water-distribution networks shut off for 24 to 36 hours on alternate days. During the winter and spring of 1997-1998, water was rationed to more than 200,000 people in northwestern Puerto Rico because water level in the Guajataca reservoir was well below normal for two years because of rainfall deficits. The drought period of 1993-1996 was comparable in magnitude to a drought in 1966-1968, but water rationing was more severe during the 1993-1996 period, indicating that water management issues such as demand, storage capacity, water production and losses, and per capita consumption are increasingly important as population and development in Puerto Rico expand.