The Sentencing Reform Act of 1984 mandated major restructuring of federal sentencing through specific sentencing guidelines. New sentencing guidelines developed by the United States Sentencing Commission and adopted in 1987 explicitly linked sentencing to “relevant conduct”—offense characteristics—and sought to abolish unwarranted sentence disparity. The guidelines substantially reduced judicial discretion and resulted in a criminalization and sentencing process that is largely prosecutor controlled. The author has generated hypotheses that relate defendant characteristics, guilty pleas, and departures from sentencing guidelines to sentence outcomes under the federal sentencing guidelines. She first examined the variables influencing sentence severity for the drug offenders who were sentenced in 1991–92. She then explored the interaction effects by estimating the tobit equation separately for three groups—black, white, and Hispanic defendants—to discover whether defendant's ethnicity conditions the effect of other defendant characteristics, guidelines-defined legally relevant variables, guilty pleas, and departures on sentence severity. Her analysis reveals that disparity in federal sentencing of drug offenders is linked not only to offense-related variables, as structured by the guidelines, but also to defendant characteristics such as ethnicity, gender, educational level, and noncitizenship, which under the guidelines are specified as legally irrelevant.