A bounded linear Hilbert space operator \(T\) is said to be \(p\)-hyponormal \((p>0)\) if \((TT^*)^p\leq(T^*T)^p\), and log-hyponormal if \(T\) is invertible and \(\log(TT^*)\leq\log(T^*T)\). \textit{J. G. Stampfli} [Trans. Am. Math. Soc. 117, 469--476 (1965; Zbl 0139.31201)] proved the following {Theorem: Let \(\lambda_0\) be an isolated point of the spectrum of a hyponormal operator \(T\) on a Hilbert space \(\mathcal H\). If \(E\) is the Riesz idempotent for \(\lambda_0\), then \(E\) is self-adjoint and \[ E{\mathcal H}=\ker(T-\lambda_0)=\ker(T-\lambda_0)^*. \] In the paper under review, the authors show that the above result holds true for the classes of \(p\)-hyponormal and log-hyponormal operators.}