The use of peak ground acceleration (PGA)-Modified Mercalli intensity (MMI) relations is discussed. A review of previous investigations in this area suggests that almost instinctively, a linear function, relating the log10(PGA) to MMI is assumed. This note proposes a quadratic form for such a relationship. The motivation for this study was provided by the application of PGA-MMI relations to the problem of damage estimations due to strong earthquake ground motion. A function relating PGA to MMI allows one to convert the probabilistic information on PGA, the typical parameter of seismic hazard analyses, to MMI, the parameter most commonly correlated to structural damage. The uncertainties in the variables used in the damage estimation are discussed.