Psoriasis is the most common T-helper cell type 1 (T(H)1) immunological disease. Evidence has linked T(H)1 diseases to myocardial infarction (MI). Psoriasis has been associated with cardiovascular diseases, but has only been investigated in hospital-based studies that did not control for major cardiovascular risk factors. To determine if within a population-based cohort psoriasis is an independent risk factor for MI when controlling for major cardiovascular risk factors. A prospective, population-based cohort study in the United Kingdom of patients with psoriasis aged 20 to 90 years, comparing outcomes among patients with and without a diagnosis of psoriasis. Data were collected by general practitioners as part of the patient's medical record and stored in the General Practice Research Database between 1987 and 2002, with a mean follow-up of 5.4 years. Adjustments were made for hypertension, diabetes, history of myocardial infarction, hyperlipidemia, age, sex, smoking, and body mass index. Patients with psoriasis were classified as severe if they ever received a systemic therapy. Up to 5 controls without psoriasis were randomly selected from the same practices and start dates as the patients with psoriasis. A total of 556,995 control patients and patients with mild (n = 127,139) and severe psoriasis (n = 3837) were identified. Incident MI. There were 11,194 MIs (2.0%) within the control population and 2319 (1.8%) and 112 (2.9%) MIs within the mild and severe psoriasis groups, respectively. The incidences per 1000 person-years for control patients and patients with mild and severe psoriasis were 3.58 (95% confidence interval [CI], 3.52-3.65), 4.04 (95% CI, 3.88-4.21), and 5.13 (95% CI, 4.22-6.17), respectively. Patients with psoriasis had an increased adjusted relative risk (RR) for MI that varied by age. For example, for a 30-year-old patient with mild or severe psoriasis, the adjusted RR of having an MI is 1.29 (95% CI, 1.14-1.46) and 3.10 (95% CI, 1.98-4.86), respectively. For a 60-year-old patient with mild or severe psoriasis, the adjusted RR of having an MI is 1.08 (95% CI, 1.03-1.13) and 1.36 (95% CI, 1.13-1.64), respectively. Psoriasis may confer an independent risk of MI. The RR was greatest in young patients with severe psoriasis.