To inform clinical management of glioblastoma patients, we estimated the relative prevalence (present at glioblastoma diagnosis) and incidence (newly diagnosed) of comorbid conditions among these patients and their matched controls. We identified 2,424 glioblastoma patients registered in the Swedish National Cancer Registry between 1993 and 2006. Next, 12,120 randomly sampled population-based controls were individually matched to cases on age, sex and calendar year of diagnosis. We then evaluated patient discharge data for selected potential comorbid conditions. Seizures (odds ratio (OR) 31.6, 95% confidence interval (CI) 24.7-40.3) and cerebral edema (OR 25.0, 95% CI 5.5-114) were the most prevalent conditions at diagnosis. Beginning 30 days after diagnosis, increased risks of incident deep vein thrombosis (hazard ratio (HR) 119.7, 95% CI 60.8-211.0) and pulmonary embolism (HR 92.4, 95% CI 48.3-176.6) were observed. Risks of incident cardiovascular diseases including heart failure (HR 4.0, 95% CI 2.6-6.1), coronary artery disease (HR 2.3, 95% CI 1.7-3.2), and myocardial infarction (HR 1.9, 95% CI 1.1-3.4) were also elevated among glioblastoma patients. In this first population-based study of both prevalent and incident comorbid conditions among glioblastoma patients, we have quantified risk of those conditions related to the tumor and its treatment-based on nationwide registry data. However, for incident conditions we cannot distinguish between the effects of the tumor and the effects of treatment. A novel finding was the elevated risk of cardiovascular disease among glioblastoma patients; glioblastoma patients should be monitored for signs of cardiovascular disease.