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In this paper, a new methodology for the design of compact planar lowpass filters in microstrip technology is proposed. This is based on cascading filter stages consisting of the combination of complementary open-loop resonators (COLR), proposed by the authors for the first time, and open stubs. By this means, we obtain sharp cutoff, compactness, and low passband insertion-loss. To illustrate the potentiality of the proposed approach, a compact S-band microstrip lowpass filter with COLR was designed, fabricated and measured.