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We present the results of multi-year gamma-ray observations by the AGILEsatellite of the black hole binary system Cygnus X-1. In a previousinvestigation we focused on gamma-ray observations of Cygnus X-1 in the hardstate during the period mid-2007/2009. Here we present the results of thegamma-ray monitoring of Cygnus X-1 during the period 2010/mid-2012 carried outfor which includes a remarkably prolonged `soft state' phase (June 2010 -- May2011). Previous 1--10 MeV observations of Cyg X-1 in this state hinted at apossible existence of a non-thermal particle component with substantialmodifications of the Comptonized emission from the inner accretion disk. OurAGILE data, averaged over the mid-2010/mid-2011 soft state of Cygnus X-1,provide a significant upper limit for gamma-ray emission above 100 MeV ofF_soft < 20 x 10^{-8} ph/cm^2/s, excluding the existence of prominentnon-thermal emission above 100 MeV during the soft state of Cygnus X-1. Wediscuss theoretical implications of our findings in the context of high-energyemission models of black hole accretion. We also discuss possible gamma-rayflares detected by AGILE. In addition to a previously reported episode observedby AGILE in October 2009 during the hard state, we report a weak but importantcandidate for enhanced emission which occurred at the end of June 2010(2010-06-30 10:00 - 2010-07-02 10:00 UT) exactly in coincidence with ahard-to-soft state transition and before an anomalous radio flare. An appendixsummarizes all previous high-energy observations and possible detections ofCygnus X-1 above 1 MeV.

Gamma-ray observations of Cygnus X-1 above 100 MeV in the hard and soft states