Hospital cleaning work, as health care work, is mainly performed by women all over Europe. Hospital cleaning activities represent a poorly studied sector although very important also for patient's health. We applied the Method of Organizational Congruencies to study cleaning work in three typical hospital units (Emergency Room, Haematology, General Medicine) of a roman University hospital where 198 women cleaners work. We analyzed the 731 technical actions performed in the three shifts and the related Organizational Constraints (OC). Working outsourcing, no occupational risks training, washing personal equipment at home, standing, long walking, early morning and night shift in emergency room, high monotony (>10 actions per hour) contact with biological and chemicals materials, risk conditions of accidents, artificial lights, hot microclimate and working in a cure setting represent the main Organizational Constraints. Differences among the three Units are discussed together with the importance of considering cleaning hospital as a preventive action towards hospital clinical risk.