From the time that a lighting installation is completed, light output begins to deteriorate. To ensure the continued effectiveness of lighting installations, earlier editions of the CIBSE Code for Interior Lighting recommended the use of planned maintenance cycles. The need for planned maintenance was reinforced in the most recent edition of the CIBSE Code, in which the recommended procedure changed from design for Standard Service Illuminance, to design for Maintained Illuminance, founded on the use of maintenance factors. Existing maintenance factors (MF) were derived from data collected in the 1950s and 1960s. A project was carried out to re-evaluate the existing values of the critically important MFs. The results of the project are reported in this paper.