[en] An abelian gauge model in 3 dimensions which includes massless scalar matter fields is analyzed. By controlling the trace anomalies with a local dilatation Ward identity, it is shown that, in perturbation theory and within the BPHZL scheme, the Chern-Simons term has no radiative corrections. This implies, in particular, the vanishing of the corresponding β function in the renormalization group equation. (author) 11 refs