A system for classification of iris color based on standard photographs, developed for use in a multicenter study, is described. Categories of iris color are distinguished based on predominant color (blue, gray, green, light brown, or brown) and the amount of brown or yellow pigment present in the iris. Two trained readers independently graded 339 iris photographs; discrepancies in grades were adjudicated. Measures of interobserver reliability were 0.76 by kappa for exact agreement and 0.97 for weighted kappa. The distribution of iris color grades demonstrates that the system achieved an appropriate level of detail within the authors' study population, which included patients with various racial backgrounds from five urban clinical centers. This simple, reliable classification system for iris color is offered for use in clinical research.