REYKJAVIK, Iceland -- In the year 874, Viking crews from western Norway started to drop in on Ireland, capture an allotment of young Celtic women and sail off northwest to a remote island beyond the reach of retribution.

Eleven centuries later, a direct descendant of those Icelandic pirates and their slave wives, Dr. Kari Stefansson, is starting to extract a tremendous prize, made possible by Iceland's tiny, isolated population and its obsessive interest in genealogy: a catalog of the deviant genes that cause the most common human diseases.

He hopes to snatch this prize away from competitors hunting for the same errant genes by a different method, including those relying on information from the $3 billion Human Genome Project.

Stefansson's 6-year-old company, Decode Genetics, says it has mapped the general location of the errant genes for 20 of the 50 common diseases on its list, including Alzheimer's, anxiety, asthma, hypertension, obesity, Parkinson's disease and rheumatoid arthritis, and has also found a region holding a longevity gene.

Within the 20 mapped locations, the Decode scientists say, they have identified specific disease-causing genes in three: those for schizophrenia and two forms of stroke.

Discovery of the variant genes that promote a disease is a first step to developing drugs that treat its root cause.

The claims of the company are hard for others to assess because so far it has published articles on only two of the disease locations it has found. But Dr. Jonathan Knowles, head of global research at Hoffmann-La Roche, said Decode's progress had been extraordinary and 13 of the disease gene locations, found under contract to LaRoche, had been verified independently for LaRoche.

Most common illnesses, including cancer, stroke, arthritis and schizophrenia, are thought to be caused by several errant genes acting together. Because each errant gene makes only a small contribution to the disease, it has no obvious pattern of inheritance and its presence is hard to spot among the natural variation in DNA sequence, making progress extremely slow.

But by focusing on Icelanders, a population of just 278,000 whose careful genealogical recordkeeping allows disease genes to be traced back more than 10 generations, Stefansson believes he has found a general method for analyzing diseases involving several genes and is well ahead in the race. "The genome isn't doing that much at the moment," he said.

The key to Decode's approach is a genealogical database that extends back 1,100 years. It has been reconstructed from a mixture of sources, including calf skins that hold the first 300 years of records, church archives and three complete censuses starting in 1703.

To look for variant genes that dispose a person to asthma, say, Decode collects the names of asthmatic patients from doctors and runs them through the genealogy database. The computer spits out family trees that are often surprisingly deep.

It is unclear whether drugs based on errant genes found in Icelanders will work elsewhere.