Cockayne syndrome (CS) is a rare degenerative disorder with a common set of symptoms but a very wide variation in phenotype. The variation is sufficiently wide that CS patients have traditionally been described in three different severity groups. Unfortunately, there is no single source for information about the different severity groups. This problem can complicate not only diagnosis, but accurate prognosis as well. The goal of this study was to describe the phenotypic variation in CS as completely as possible. This article provides extensive descriptions of traits common to each group and their degree of severity in each group. Forty-five people with CS were surveyed and information from the published literature was used to increase the sample sizes for calculations. This study provides new information, including statistical data for each of the three severity groups (mean age at death, average head circumference, and average length or stature). The study includes cerebro-oculo-facial syndrome (COFS) as a severe form of CS, based on results of recently published genetic studies performed by other authors. This study proposes revised names for CS severity groups: severe, moderate, and mild. The groups were formerly called Type II/early onset CS, Type I/classical CS, and Type III/atypical/mild/late-onset CS, respectively. A fourth newly documented group, UV sensitivity only/adult onset, is also described. Average ages of death were calculated as 5.0 years (severe), 16.1 years (moderate), and 30.3 years (mild). © 2011 Wiley-Liss, Inc.