We have realized an innovative integrated detector for electron spin resonance spectroscopy. The microsystem, consisting of an LC oscillator, a mixer, and a frequency division module, is integrated onto a single silicon chip using a conventional complementary metal-oxide-semiconductor technology. The implemented detection method is based on the measurement of the variation of the frequency of the integrated LC oscillator as a function of the applied static magnetic field, caused by the presence of a resonating sample placed over the inductor of the LC-tank circuit. The achieved room temperature spin sensitivity is about 10(10) spins/GHz(1/2) with a sensitive volume of about (100 mu m)(3). (C) 2008 American Institute of Physics. [DOI: 10.1063/1.2969657]