As patients live longer, and more illnesses become chronic, comorbidity rates increase. This increase shows in attended morbidity registers, and its importance is revealed by the use of services, treatments, the survival rates and the patients life quality. A frequent coexistence of illnesses, such as diabetes and depression, increases the cost of primary care and emergency treatments and the number of hospital admissions; increases the probability of cardiovascular risk and non-adherence to treatment and diet; causes a perception of poor physical and mental health and, on the whole, increases global costs. The annual mortality rate shoots up when anemia and cardiovascular or kidney failure occur. Mental comorbidity has a greater effect on the patients perception of their life quality than physical comorbidity. Comorbidity increases the number of hospital admissions which could have been prevented, as well as the avoidable complications related to them. Suicidal thoughts occur more frequently in patients suffering from chronic obstructive pulmonary disease (COPD) than in those suffering from other chronic organic illnesses. In patients suffering from atrial fibrillation, the risk of acute coronary crisis increases as they age and the comorbidity increases the use of angiotensin-converting enzymes (ICAE), antidepressant drugs and the visits to the psychiatrist.