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We investigate the effect of uncertainties in the cross- sectional parameters of ungrounded coplanar waveguides (CPWs) on the characteristic impedance and the propagation constant. For our calculations, a quasi-TEM model is used that takes into consideration the effects of non-ideal conductors, substrate loss and finite metallization thickness. The propagation of uncertainties is studied with the aid of Monte Carlo (MC) simulations. The effect of different manufacturing technologies is exemplified by two typical microwave substrates (AF45 and AI2O3). Furthermore, the impact of the uncertainties in the input quantities is investigated in such a way that conclusions for the design of CPWs as well as for the requirements on the accuracy of material measurement methods can be drawn.