Ordered Large according to my measurement, 42".  Fit very sloppy and large for a "slim" fit.  Even my wife, who couldn't care less about this sort of thing, said, that's too big.  And the quality was poor, thin, nearly sheer.  Disappointing considering its from CL.  I have on a Polo RL shirt, same thing, slim fit Large and it is exactly what I expected.  Soft, quality proper slim fit.