Even though the term new factor is poorly defined, Wisconsin trial courts have discretion to modify a defendant's criminal sentence if the defendant introduces a new factor. All sentence modification motions filed by the Legal Assistance to Institutionalized Persons program from 1978 through the summer of 1987 were studied. Trial courts modified sentences under the theory of new factor in a variety of cases to give effect to their sentencing intent when it had been frustrated, to react to a change in a defendant's health or family circumstances, or to reward a defendant for cooperation with prosecutors after sentencing. Wisconsin's common law sentence modification system is confusing at the appellate level, but functions well to provide defendants with a broader and more flexible form of post-conviction relief than is available in States where statutes define the terms of sentence modification. 94 footnotes. (Author abstract modified)