Background Informal caregiving offered by family members has been widely studied in the community setting, but little attention to date has been dedicated to that offered at the hospital level. Aims To describe the proportion of patients admitted to acute medical units receiving care from informal caregivers as decided by the family and to identify the factors affecting the numbers of care shifts performed by informal caregivers. Design and methods A longitudinal study was performed involving 12 acute medical units located in 12 northern Italian hospitals. Results All patients (N = 1464) admitted to medical units were included, and 77.1% of them received at least one shift of informal care during their in-hospital stay, especially during the mornings and afternoons. At the patient level, those at higher risk of prolonged hospitalisation and difficult discharge at admission, and those reporting higher occurrence of adverse events, such as pressure sores, confusion events and use of physical restraints during hospitalisation, were more likely receiving informal care. At the nursing care level, a higher amount of missed nursing care was associated with an increased number of care shifts offered by informal caregivers during patient hospitalisation, whereas the amount of care offered by staff was a protective factor. Conclusions Families play a care role in the care of older patients admitted to acute medical units. They contribute substantially to the care of patients, especially during morning and afternoon shifts.