We systematically and quantitatively investigated the structure and thermodynamics of G-quadruplexes of RNAs and corresponding DNAs of the same sequences under molecular crowding conditions that mimic the high osmotic stress induced by the numerous molecules inside of living cells. Structural analyses demonstrated that various telomere RNA sequences folded into parallel-stranded G-quadruplexes in a manner independent of the surrounding conditions with different cations under both dilute and molecular crowding conditions. In contrast, DNA G-quadruplexes showed structural polymorphism. Moreover, we demonstrated that the G-quadruplexes of the RNA sequences were more stable than those of the same DNA sequences. These results show that a single and robust RNA G-quadruplex structure can exist in a manner independent of the sequence and surrounding conditions. To confirm this, we studied a guanine-rich sequence located in the 5'-untranslated region of human bcl-2 mRNA that is thought to play a role in translation. The results revealed a stable parallel G-quadruplex that formed under all conditions tested. For example, a bcl-RNA G-quadruplex in the presence of 5 mM KCl [free energy change at 25 degrees C (DeltaG degrees (25)) of -5.42 kcal/mol] was more stable than its corresponding DNA G-quadruplex (DeltaG degrees (25) = -2.31 kcal/mol). Our results further indicated that water molecules binding to the 2'-OH group of RNA G-quadruplexes play a critical role in their formation and stability.