The modification of the adhesion and spreading of BKK21 fibroblast cells has been studied for new biocompatible surfaces obtained by irradiating polyhydroxymethylsiloxane thin films with increasing doses of 5 keV Ar ion beams. The irradiated surfaces showed a dose-dependent increase of cytocompatibility, with an observed onset of the effect at about 5 x 10(14) ions/cm(2). At this dose, in fact, we found both the enhancement of cell adhesion, for an incubation time of 2 h, and complete cell confluence after an incubation time of 48 h. The observed fluence-dependent trends in cell adhesion and spreading have been correlated with the irradiation-induced modifications of the polymer surface composition and the related change in surface energy, obtained by using X-ray photoelectron spectroscopy (XPS) and contact angle measurements of three liquids. XPS data showed that ion irradiation induced a progressive compositional modification of the polymer toward a SiOx-rich phase, because of the irradiation-induced formation of [SiO4] clusters and decrease of the original [SiO3-C] ones, involving the loss of more than 50% of the original methyl groups and the transformation of the residual carbon-containing groups in a dispersed hydrogenated amorphous carbon phase of nanometric size. The surface free energy measurements, performed with the static contact angle technique, showed that ion irradiation transforms the initially hydrophobic surfaces, with theta = 77.6 degrees +/- 1.5 degrees, into much more hydrophilic ones, with theta = 31.4 degrees +/- 1.7 degrees. Furthermore, the contact angle is found to undergo an abrupt decrease just at an ion dose of 5 x 10(14) ions/cm(2), that is, where the onset of cell adhesion and confluence is observed. The analysis of the observed changes in the total surface energy in terms of the relative polar and dispersive force contributions showed that the strong enhancement of the hydrophilic character of the irradiated surfaces is mainly due to the raising of the polar acid-base force components, this effect being due to the enrichment of the irradiated surfaces with the permanent dipoles of the [SiO4]-based network and the elimination of the original pendant methyl groups.