X-ray diffraction analysis of (R,S)- and S-atenolol crystalline forms was performed. The crystals studied were grown from evaporation of an ethanol/water solution. (R,S)-Atenolol crystallizes in the centrosymmetric space group C2/c, and S-atenolol crystallizes in a noncentrosymmetric space group C2. There is one symmetry independent molecule in (R,S)-atenolol crystals and two symmetry independent molecules in S-atenolol. However, due to disorder, two different molecular conformations were identified in the (R,S)-atenolol, and three different conformations were isolated in S-atenolol. Flexibility of molecular segments of the carbon chain is seen in conformational isomorphism and in the atomic position uncertainty. The molecular conformations given by X-ray diffraction were fully relaxed at the HF/6-31G* level of theory. The optimized structure was used as reference in comparison with molecular conformation in the solid state. ; http://dx.doi.org/10.1021/cg0601857