  The tiny general relativistic Lense-Thirring effect can be measured by means
of a suitable combination of the orbital residuals of the nodes of LAGEOS and
LAGEOS II and the perigee of LAGEOS II. This observable is affected, among
other factors, by the Earth' s solid and ocean tides. They induce long-period
orbital perturbations that, over observational periods of few years, may alias
the detection of the gravitomagnetic secular trend of interest. In this paper
we calculate explicitly the most relevant tidal perturbations acting upon
LAGEOSs and assess their influence on the detection of the Lense-Thirring
effect. The present day level of knowledge of the solid and ocean tides allow
us to conclude that their influence on it ranges from almost 4% over 4 years to
less than 2% over 7 years.
