IN THIS SEMINAR PAPER, THE NEW MEASURE IS TERMED CRIME-DAYS PER YEAR AT RISK. A CRIME DAY IS A 24-HOUR PERIOD IN WHICH AN INDIVIDUAL COMMITS ONE OR MORE CRIMES. THE NUMBER OF CRIME-DAYS PER YEAR AT RISK REFERS TO THE NUMBER OF DAYS PER YEAR THAT AN INDIVIDUAL HAS COMMITTED CRIMES, FROM 0 TO 365. YEARS AT RISK REFERS TO TIME WHEN THE SUBJECT WAS NOT INCARCERATED. THE NEW MEASURE PERMITS THE CALCULATION OF UNIFORM CRIME RATES BY YEARS AT RISK AND IS NOT CONFOUNDED BY MULTIPLE CRIMES COMMITTED ON A GIVEN DAY. DATA WERE OBTAINED FROM A RANDOM SAMPLE OF 243 OUT OF 4,069 MALE BALTIMORE OPIATE ADDICTS LISTED BY THE POLICE BETWEEN 1952 AND 1971. ALTHOUGH COMPREHENSIVE INSTITUTIONAL DATA WERE COLLECTED ON THE SAMPLE, TRAINED INTERVIEWERS FAMILIAR WITH THE BALTIMORE ADDICT SUBCULTURE OBTAINED THE PRIMARY ANALYSIS DATA IN 3-HOUR INTERVIEWS. QUESTIONS FOCUSED ON SIX TOPICS; DRUG USE, CRIMINAL BEHAVIOR, WORK, LIVING ARRANGEMENTS, DRUG SELLING, AND OTHER SOURCES OF INCOME. THE FOLLOWING DATA WAS CODED FOR EACH SUBJECT: (1) TOTAL CRIME-DAYS WHILE ADDICTED, (2) TOTAL CRIME-DAYS OFF OPIATES WHILE 'ON THE STREET', (3) CRIMELESS DAYS WHILE ADDICTED, AND (4) CRIMELESS DAYS WHILE OFF REGULAR OPIATE USE. THE RESULTS SHOW THAT MOST OF THE SUBJECTS WERE DEEPLY ENMESHED IN CRIMINAL CAREERS ON A DAILY BASIS OVER A PERIOD OF MANY YEARS. ON AN ANNUAL BASIS, EACH ADDICT COMMITTED AN AVERAGE OF 178 CRIMES PER YEAR SINCE THE ONSET OF REGULAR OPIATE USE. WITH RESPECT TO YEARS AT RISK, 91.4 PERCENT OF THEIR CRIME DAYS WERE ALSO DAYS DURING WHICH THE SUBJECTS WERE ADDICTED; CONVERSELY, ONLY 8.6 PERCENT OF THEIR CRIME-DAYS WERE ABSTINENT DAYS. THEFT AND THE SALE OF DRUGS WERE THE MOST FREQUENT CRIMES. THE RESEARCH FINDINGS ARE CONSISTENT WITH THOSE OF OTHER STUDIES. TABLES AND REFERENCES ARE INCLUDED. ANALYSIS OF COHORT AND RACE DIFFERENCES WAS UNDERTAKEN ELSEWHERE.