Typical 1-D velocity models of anomalies in the lowermost mantle show a positive velocity jump of 2--3% in the bottom few hundred kilometers of the mantle, the core-mantle transition zone. With synthetic seismograms it is demonstrated that a thin lamella of about 20 km thickness in D' can produce similar seismic signals, for P and for S waves. A reduction of the thickness and/or velocity contrast of this lamella will render it seismically invisible. The intermittent detection of anomalies in the lower mantle could therefore be due to the intermittent existence of inhomogeneities or, if lamellae exist in D', be produced by variations in their thickness, velocity contrast and/or undulation. This demonstrates the uncertainty in our present knowledge of the D' structure. ¿ American Geophysical Union 1994