Aims The purpose of the current work was to study the pattern and dynamics of biofilm formation in clinical isolates of Staphylococcus aureus and Staphylococcus epidermidis in the presence of 10 antibiotics with different action mechanisms. Methods and Results By using impedance measurements in microtitre plates with gold electrodes we have assessed the antibiotic effect on bacterial biofilm growth in real time. The impedance measurements appear to combine both cellular growth and matrix production, representing a measurement of total biofilm mass. Several clinical and reference strains were tested, showing different slopes and cell index values which correlated with their capacity to form biofilms as assessed by attachment to standard microtitre well plates and safranin staining. Biofilms were heavily reduced in biofilm mutants or by protease treatment in protein-based biofilm matrixes. Antibiotic resistance patterns of biofilms, which were very different to those obtained by traditional methods like epsilon-tests on solid media, revealed features that would pass unnoticed by end-point methods. Conclusions Once the biofilm is formed, antibiotic efficacy dramatically reduced and sub-inhibitory concentrations of some antibiotics, such as linezolid and clarithromycin, stimulated biofilm growth, stressing the importance of studying antibiotic resistance under biofilm growth conditions in real time. Significance and Impact of the Study Real-time biofilm analysis provides a promising tool to evaluate antibiotic therapy in clinical biofilm-mediated infections.