Patients with the skin disease psoriasis may be at an increased risk of developing the immune system cancer lymphoma.
Psoriasis can be disfiguring
Researchers from the University of Pennsylvania found lymphoma was almost three times as common among people with psoriasis.
It is not known whether the condition itself, or the treatments increase the risk of cancer.
The research is published in Archives of Dermatology.
Psoriasis causes the skin to become very inflamed, producing red, thickened areas with silvery scales, mainly on the scalp, elbows, knees and lower back.
It can be painful, and even disfiguring in some cases.
Lymphoma is a group of cancers affecting the lymph tissues found mainly in the lymph nodes and spleen.
The researchers studied a random sample of 108,000 patients aged 65 or older.
During the study period, they identified 276 lymphomas. Analysis revealed that an extra 122 cases of lymphoma per 100,000 patients a year were found among psoriasis patients.
The researchers say more work is needed to ascertain whether developing psoriasis is in itself enough to increase the risk of lymphoma, or whether it is only linked to severe forms of the disease.
They say it is possible that the risk comes from the treatments used on psoriasis.
Previous research has found an increased risk of cancer among psoriasis patients treated with a combination of photochemotherapy and the immunosuppressant drug cicolosporin.
Alternatively, it be the result of an interaction between the two risk factors.
Dr Jane McGregor, skin cancer expert from Cancer Research UK, told BBC News Online that even though psoriasis patients seemed to be at increased risk, the overall risk was still tiny.
She said: "It is unlikely that psoriasis itself is responsible for the increased risk as there is no obvious link.
"However, we know patients with severe psoriasis are often treated with drugs that suppress the immune system, and these have been associated with an increased risk of lymphoma."
Dr McGregor highlighted on drug in particular called Razaxone which although an effective treatment for psoriasis, was withdrawn in the UK several decades ago amid fears about its safety.
Gladys Edwards, of the Psoriasis Association, said: "Clearly we are concerned about the results of this research.
"We would welcome further research both to broaden the database and to clarify the risk factors.
"This would help to determine a course of action to benefit patients whether that is an improved screening service or clearer information about treatments and their side effects."