Intrinsic disorder refers to segments or to whole proteins that fail to self‐fold into fixed 3D structure, with such disorder sometimes existing in the native state. Here we report data on the relationships among intrinsic disorder, sequence complexity as measured by Shannon's entropy, and amino acid composition. Intrinsic disorder identified in protein crystal structures, and by nuclear magnetic resonance, circular dichroism, and prediction from amino acid sequence, all exhibit similar complexity distributions that are shifted to lower values compared to, but significantly overlapping with, the distribution for ordered proteins. Compared to sequences from ordered proteins, these variously characterized intrinsically disordered segments and proteins, and also a collection of low‐complexity sequences, typically have obviously higher levels of protein‐specific subsets of the following amino acids: R, K, E, P, and S, and lower levels of subsets of the following: C, W, Y, I, and V. The Swiss Protein database of sequences exhibits significantly higher amounts of both low‐complexity and predicted‐to‐be‐disordered segments as compared to a non‐redundant set of sequences from the Protein Data Bank, providing additional data that nature is richer in disordered and low‐complexity segments compared to the commonness of these features in the set of structurally characterized proteins. Proteins 2001;42:38–48. © 2000 Wiley‐Liss, Inc.