SAN JUAN ALOTENANGO, Guatemala -- In 2004, Ambrosio Carrillo made a perilous and illegal journey to the U.S. in search of opportunity. Earlier this year, he made the equally wrenching decision to return home.

Once a construction worker earning about $15 an hour in Maryland, Mr. Carrillo barely worked in the fall of 2007 as plentiful jobs evaporated. As winter set in, the illegal immigrant, who had mastered masonry, carpentry and drywalling in the U.S., didn't land a job for two months. There was no money to send to his wife and three children in Guatemala.

So in January, Mr. Carrillo sliced open the green plastic piggy bank he'd bought at Wal-Mart and counted $3,100 in change and bills. "There was enough to buy a plane ticket home and ship my truck to Guatemala," recalls Mr. Carrillo, 37 years old. Now back in San Juan Alotenango, a town of dirt streets and sporadic running water, he hauls fruit, firewood and recyclable metal for a few dollars a trip.

With his journey to the U.S. and back, Mr. Carrillo is helping to write the latest chapter in the American immigrant story. After years of growth, illegal immigration to the U.S. from Mexico and Central America has slowed sharply. At the same time, say demographers and immigrant advocates, more Latin American immigrants like Mr. Carrillo are apparently returning home. The impact of this shifting migration pattern is felt in the U.S. and beyond, in towns like San Juan Alotenango that depend to some degree on cash sent home by those working in the U.S.

It is difficult to track short-term changes in the population of the estimated 12 million immigrants who are in the U.S. illegally. But a new study by the Pew Hispanic Center, an independent think tank in Washington, D.C., estimates that annual undocumented arrivals from Mexico are down about 25% this year from 2005, to about 350,000. Undocumented arrivals from Central America have been halved since then, to about 120,000, according to the study, which is due to be released Thursday.