It is not unusual for paediatric surgical patients to suffer from paediatric diseases in addition to their surgical problems. These diseases demand further diagnostic procedures and pre- or postoperative therapy. The aim of this study was to discover how many and what kinds of additional paediatric diseases are seen in our paediatric surgical inpatients. We retrospectively evaluated the hospital charts of all inpatients for 1 year. The following data were collected: cause of admission, therapeutic procedure (conservative/operative), surgical discharge diagnosis, additional paediatric diagnoses, and transfer to other departments. A total number of 5,026 hospital stays for 5,840 operations in 4,300 children was evaluated. In 38% of all hospital stays, the children had one or more paediatric diseases. These could be divided into two groups: acute diseases and chronic conditions. In the acute group, 638 children suffered from acute infectious diseases such as respiratory infections and enteritis/gastroenteritis. The most common chronic conditions were allergy, asthma, epilepsy, anaemia, and mental retardation. A special group of patients consists of 21% of the children admitted because of suspected appendicitis. These children not only had a significantly higher incidence of additional paediatric diseases (mostly acute infectious diseases), but also a higher incidence of atopic diseases compared with the patients admitted for other reasons. The early diagnosis and treatment of additional paediatric diseases is essential for the success of paediatric surgery. The paediatrician has a significant role in caring for surgical patients and assisting in the perioperative management, but the paediatric surgeon should also be aware of the spectrum of medical diseases.