It has been speculated that the initial bending of cantilevers has a major influence on the detector signal in a cantilever-based sensor using the optical lever readout method. We have investigated theoretically as well as experimentally the changes induced in the detector signal when the optical lever technique is used to monitor a cantilever with initial bending during changes in the refractive index of the surrounding media. We find that for changes in refractive index as small as 10−4 the detector signal is highly dependent on the initial bending of the cantilever. The findings are validated experimentally using an environmental chamber and varying the pressure. We sketch routes to circumvent the problem and formulas suitable for data treatment are given.