Situated about 900 km southwest of mainland Portugal, in the North Atlantic, Madeira is the bigger and more populated island of the archipelago with the same name. It has a total area of 747 km2 and its northern slope forms a barrier that opposes the prevailing north-easterly trade winds, thus resulting in a very frequent windward fog belt, between 800-1600 a.s.l. Madeira has a 125 km2 area of indigenous altitude forests inside the windward fog belt, between 800-1600 a.s.l. (Prada et al., 2008). This area is characterized by very steep slopes, mainly exposed to the prevailing winds. When combined, factors like steep slopes, great exposure to the humid trade winds and presence of forest vegetation facilitate fog precipitation. This is why we assume that fog precipitation is a generalized phenomenon throughout Madeira’s northern slope area. To ascertain whether or not fog water contributes to groundwater recharge, a study on the stable isotopic composition was made. For that purpose, assuming a difference between isotopic composition in rain and fog (fog being enriched in heavier isotopes 2H and 18O relative to rain at the same altitude and region) (Ingraham and Mathews 1988, 1990; Clark and Fritz, 1997), several samples of fog water, rain water and groundwater were collected for stable isotopic analysis. Groundwater was collected, according to Clark and Fritz, (1997), from springs and tunnels representing perched and basal aquifers, respectively; fog water was collected on trees by hand by placing a funnel in a collection bottle and dabbing droplets which collected on the foliage, in rainless days of intense fog, under 98-100% relative humidity conditions, thus preventing sample evaporation enrichment; rain water was collected in containers with a 1 cm thick layer of mineral oil to prevent evaporation according to Clark and Fritz (1997) and School et al., (2002), representing a sample of several rain events. The preliminary stable isotopic compositions of fog, rain and groundwater samples collected are plotted in figure 1 along with the Global Meteoric Water Line (GMWL), first described by Craig (1961). The rain samples are the most isotopically depleted and fog samples the most enriched ones. Groundwater data is plotted in an intermediate position between the stable isotopic ratios for rain and fog waters. The composition of groundwater may be explained simply in terms of evaporation of the rain prior to infiltration. However, the isotopic composition of groundwater does not appear to depict any evaporative effects. So, the best explanation is that some of the fog water infiltrates and recharges the groundwater system of the Madeira Island, as observed in other regions by Ingraham and Matthews (1988; 1990; 1995).