When Janet Riordan returned from a European vacation in January, she knew immediately that something was wrong with her 7-year-old golden retriever Reggie. She said that he appeared to be sobbing. A visit to the veterinarian's office confirmed it: Reggie had an aggressive form of lymphoma, a cancer of the white blood cells. The disease was all too familiar to Ms. Riordan, who had lost her father to the disease just four years earlier.

"I never thought I would lose my dad and my dog to the same disease," Riordan said.

Pet owners share their homes, their television viewing habits and sometimes their beds with their dogs. And now, increasingly, researchers are reporting that they are sharing the same illnesses: obesity, diabetes, heart disease, cancer and asthma.

Doctors and veterinarians are working together to find pollutants and other environmental factors that may play a role in the disease, like pesticides, cigarette smoke and household chemicals.

Pets won't be able to provide the be-all-end-all of evidence of a link between environmental factors and certain diseases, but it's a start. As heartbreaking as it is when companion animals suffer, pets make great models for people when researchers want to study diseases. Their lives are like abbreviated versions of humans, and the diseases resemble their appearance in humans. And, because veterinarians are familiar with their breed history, it is relatively simple to distinguish how genetics and environment interact.

Researchers have already started to put together clues. Like many young children, cats and dogs have increased exposure to lawn and garden pesticides and household chemicals lying in the dust or in carpet. Lawn care chemicals can increase the risk of canine lymphoma and bladder cancer. Cats who receive exposure to flame retardants have a higher risk of thyroid cancer. And researchers have begun to find similar links in humans: people exposed to high levels of DDT and other pesticides later developed non-Hodgkin's lymphoma at high rates.

The University of Massachusetts and Tufts University Cumming School of Veterinary Medicine questioned the owners of more than 700 dogs of their use of pesticides. Of these dogs, a third had malignant lymphoma, a third had benign tumors, and a third was receiving surgery for other reasons. Dogs whose owners used professionally applied pesticides were 70 percent more likely to later have lymphoma.

Another study, The Golden Retriever Study, aims to see how many environmental chemicals may interact with genes in a breed already predisposed to health problems. Likened to the Nurse's Health Study, which tracks women's health, the study will enroll 3,000 golden retrievers and follow them throughout their lives.

In Reggie's case, Janet Riordan looked to human experience for treatment. Her veterinarian gave Reggie chemotherapy and experimental high-dose Vitamin C treatments. In the end, it was too late. Reggie died two months later, and Riordan never did find out if anything was to blame for his disease. They never used pesticides and golden retrievers do have a high rate of developing cancer.

"We love them so much that even if they don't die of cancer, they will ultimately break our hearts," she said.