We study 46 solar coronal eruptions associated with sigmoids seen in images from the Yohkoh Soft X-ray Telescope (SXT). We relate the properties of the sigmoids to in situ measurements at 1 AU and geomagnetic storms. Our primary result is that erupting sigmoids tend to produce geoeffective magnetic clouds (MCs): 85% of the erupting sigmoidal structures studied spawned at least a moderate (|Dst| ≥ 50 nT) geomagnetic storm. A collateral result is that MCs associated with sigmoids do not show the same solar-terrestrial correlations as those associated with filaments and, as such, form a distinct class of events. First, rather than reversing with the global solar dipole (at solar maximum), the leading field in MCs weakly (2:1) shows a solar cycle (Hale polarity) based correlation (reversing at solar minimum). Second, whereas the handedness of MCs associated with filament eruptions is strongly (95%) related to their launch hemisphere, that of MCs associated with sigmoid eruptions is only weakly (~70%) so related. Finally, we are unaware of any model of the magnetic fields of sigmoids and their eruption that gives a useful prediction of the leading field orientation of their associated MC.