The first heat-flow density values acquired on the Balearic Islands are presented. A total of 39 thermal gradient determinations were carried out, 23 on Mallorca and 16 on Menorca. The sampling points consist of water exploration wells whose depths range between 100 m and 250 m. Most of the thermometric logs are strongly affected by water circulation through the formation and/or through the borehole. Heat-flow density measurements have been performed from 18 boreholes where conductive heat transport is inferred. Thermal conductivity values were obtained from 52 available well core samples covering the main lithologies present in the area. After topographic and paleoclimatic corrections, surface heat-flow values ranging from 24 ± 13 mW/m2 to 88 ± 23 mW/m2 for Mallorca and from 46 ±6 mW/m2 to 92 ± 20 mW/m2 for Menorca are obtained. A well-defined positive thermal anomaly associated to upward ground-water flow is described on Mallorca. Water flowing through the Mesozoic basement would be responsible for an overall cooling effect affecting the northeastern half of the island where the lower heat-flow values are measured. Water perturbations on Menorca are mainly due to hydraulic connection through the borehole. Taking into account the effects of the regional ground-water flow, background heat-flow densities of about 70-80 mW/m2 for Mallorca and 75-90 mW/m2 for Menorca are proposed.