It is well know that most spiral galaxies need an additional, invisible component to fit their observed rotation curve. Persic et al (1996) model the rotation curve of galaxies as a function of a single parameter: their luminosity. Fainter galaxies exhibit rotation curves that requires more dark matter when compared to the more luminous ones. In this work I investigate a relationship between the amount of dark matter in spiral galaixes and the HI radio emission at 21 cm.