It has long been suspected that FAD metabolic FLIM data are biased by a fluorescence decay component from FMN. We therefore checked for possible traces of FMN fluorescence in FLIM data from our DCS-120 metabolic FLIM system. Indeed, we found a decay component with a lifetime around 4.8 ns which cannot be attributed to FAD but very well to FMN. The amplitude of the FMN component was 3.9% in normal cells and 8.8% in cancer cells. The presence of a decay component from FMN may have implications to the use of FAD data for cancer diagnosis, especially if the amplitude, a1, the amplitude ratio, a1/a2 or the FLIRR ratio are used as cancer indicators.