The use of aseptic instruments for the care of patients is an essential element in the prevention of nosocomial infections. Significant risks have been associated with inadequate or improper cleaning and disinfection of reusable medical devices. Thermal disinfection with moist heat, based on the A0 concept (EN ISO 15883-1), is the most common method for disinfection of medical devices in the hospital setting. A0 is a physical parameter denoting the inactivation of microorganisms. The concept of A0 is intended to allow equivalent disinfection efficiencies to a reference time/temperature to occur at other disinfection temperatures. This paper focuses on parametric control of thermal disinfection--A0 values as recommended in the standard and their interpretation. The experimental fundamentals regarding of an A0 concept are rare. Data on thermal disinfection are partly contradictory. The washer disinfectors use thermal disinfection programs set in accordance with the parameters: time and temperature, which is proven suitable biocidal activity, not based on the A0 value. Many authorities in the field of disinfection recommends to use higher values of A0 than those specified in the standard EN ISO 15883.