This study evaluated the effectiveness of mandatory sentencing by determining the number of felonies prevented if these sentences had been in operation since 1975. Using records of the Chief Probation Officer of Suffolk County, 276 cases were randomly selected from a list of offenders convicted of serious crimes in 1975. The criminal history of each offender was disaggregated into those felonies which occurred prior to the 1975 offense and felonies and misdemeanors which took place after it. The sample consisted primarily of young, single, black men. The majority convicted in a given year were career offenders. Over 60 percent had been convicted of a major offense prior to the 1975 conviction, and over 40 percent had been convicted for at least one offense since 1975. There was a correlation between prior and subsequent offenses. Incapacitation had more of an effect on reducing property crimes than violent offenses, but the effects of an incapacitation policy on the crime rate were of an incapacitation policy on the crime rate were negligible. In addition, mandatory sentencing would produce at least a 61-percent increase in prison population. Tables, charts, and approximately 20 references are supplied.