Geometric analysis of 33 refined high-resolution protein crystal structures (2 A or higher) demonstrates that side-chain amino groups interact with aromatic side chains. Positively charged or delta(+) amino groups of lysine, arginine, asparagine, glutamine and histidine are preferentially located within 6 A of the ring centroids of phenylalanine, tyrosine and tryptophan, where they make van der Waals' contact with the delta(-) pi-electrons and avoid the delta(+) ring edge. This geometric pattern is different from the distribution expected due to random close packing of side chains in a protein. It is opposite to oxygen- and sulfur-aromatic interactions, similar to aromatic-aromatic interactions, and almost certainly electrostatic in origin.