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Transformer-based variable inductors usable up to ~77 GHz are demonstrated. Measured inductance of two variable inductors can be tuned from 56.5 to 67 pH and from 55.7 to 61.9 pH at 77 GHz. The bandwidth Q is greater than ten at 77 GHz. This is higher than that of typical Q of ~4 for MOS varactors which is an alternate tunable element. This indicates that variable inductors with useful tuning range and sufficiently high Q can be realized in CMOS for use in millimeter-wave circuits.