We examine the contribution of W boson loops to the amplitude of the p
rocess H --> gamma gamma within the dispersion relation approach, taki
ng up an issue raised in this context recently. We show that the non-v
anishing limit of the relevant form factor for m(w) --> 0 is due to a
finite subtraction induced by the value of the corresponding trace ano
maly. The argument can be turned around and one thus arrives at a disp
ersive (''infrared'') derivation of the trace anomaly.