The Hospital Morbidity Survey (HMS) performed in hospitals is a sample study that began in 1951 and has been carried out annually since then. Its main objective is to ascertain the structure and development of hospital morbidity, that is to say, all persons admitted to a hospital centre for treatment as in-patients. In particular, the Survey aims to measure morbidity according to the main or discharge diagnosis, the average hospital stay by type of diagnosis, the scope of influence or degree of attraction of hospitals and to act as a starting point for in-depth studies on certain diseases.

It was originally obligatory only for public hospitals to take part in the Survey, with the participation of private centres being voluntary, but the Survey was reviewed in 1977 and became a national Survey, including all Spanish public and private hospitals.

The fulfilment of the Survey's objectives was guaranteed with the introduction of the obligatory use of basic health documents in 1977: the Patient Register and the Patient File, which granted the systematic recording of all patients who were admitted to and discharged from hospital.

The classification variables of the Survey consist of the main diagnosis and the demographic and clinical circumstances of the patient (sex, age, date of admission and discharge, province of residence, type of admission, reason for discharge). This allows us to understand morbidity in hospitals in a certain reference year and makes information available on a state level on the availability and use of hospital resources. From 2011 on, information broken down has been included to the publication according to the Health dependency, showing information about public and private hospitals separately

Currently, the collection of necessary information in hospitals for the Survey is mixed, as the information comes from the two aforementioned base documents: the BMHDS, for both public and private hospitals, and the Record Book, mainly for those centres that lack computerised monitoring of patients.