In this paper a $T(1)$ theorem for Calder\'on--Zygmund
operators (CZOs) valid for non-doubling measures with
atoms is proved. In the classical Calder\'on--Zigmund
theory an essential assumption is the doubling property
of the underlying measure $\mu$ in ${\mathbb R}^n$.
Some recent results have shown that a big part of the
classical theory can be extended to the case of
non-doubling measures, with only some mild `growth'
condition on the measure $\mu$.
In the particular case of the Cauchy transform
$Cf(x) = \int f(y)/(y-x) \, d\mu(y)$, the main result of
the paper can be stated easily: the Cauchy transform is
bounded in $L^2(\mu)$ if and only if it is bounded in
$L^2(\mu)$ over characteristic functions of squares.
The proof of the main result is based on the use of
the Haar basis with random dyadic lattices, following
an idea of Nazarov, Treil and Volberg (`Cauchy integral
and Calder\'on--Zygmund operators on nonhomogeneous
spaces', {\em Internat. Math. Res. Notices}
(1997) 15, 703--726), where a $T(1)$ theorem for
non-doubling continuous measures was obtained.
The case of a measure $\mu$ with atoms studied in the
present paper is quite different from the case of
continuous measures. For example, if $\mu$ has atoms,
the $L^2(\mu)$ boundedness of a CZO does not imply its
$L^p(\mu)$ boundedness for some $p\neq2$. The differences
arise because a measure $\mu$ with atoms does not
satisfy the essential `growth' condition that is involved
in the arguments for proving the $L^2$ boundedness of
CZOs for continuous measures. 2000 Mathematics Subject Classification:
42B20.