['To determine if the rate of lymphoma in patients with a history of psoriasis is different from the rate of lymphoma in patients without psoriasis.', 'Cohort study.', 'Outpatient practices of general practitioners in the United Kingdom who contribute to the General Practice Research Database.', 'The population studied was a sample of 10% of the patients 65 years or older registered with a general practitioner contributing to the General Practice Research Database between 1988 and 1996.', 'The rate of lymphoma in patients with psoriasis compared with the rate of lymphoma in patients without psoriasis.', 'There were 2718 patients who had psoriasis and 105 203 patients (the reference population) who did not have psoriasis. The median follow-up time was 46 months. We noted 276 lymphomas. Patients with psoriasis had a 2.95 relative rate of developing lymphoma (95% confidence interval, 1.83-4.76) compared with those without psoriasis. This estimate did not change after controlling for age and sex using the Cox multivariable proportional hazards model. The rate of lymphoma changed little when the patients treated with methotrexate or those who developed mycosis fungoides were excluded. Compared with the reference population, we found an additional 122 lymphomas per 100 000 patients annually among patients with psoriasis who were 65 years or older.'] Are lymphoma rates low but increased in patients with psoriasis : results from a population-based cohort study in the United Kingdom?