OBJECTIVE: To determine the use effectiveness of the Creighton model ovulation method in avoiding and achieving pregnancy. DESIGN: Prospective, descriptive. SETTING: A natural family planning clinic at a university nursing center. PARTICIPANTS: Records and charts from 242 couples who were taught the Creighton model. The sample represented 1,793 months of use of the model. MAIN OUTCOME MEASURE: Creighton model demographic forms and logbook. RESULTS: At 12 months of use, the Creighton model was 98.8% method effective and 98.0% use effective in avoiding pregnancy. It was 24.4% use effective in achieving pregnancy. The continuation rate for the sample at 12 months of use was 78.0%. CONCLUSION: The Creighton model is an effective method of family planning when used to avoid or achieve pregnancy. However, its effectiveness depends on its being taught by qualified teachers. The effectiveness rate of the Creighton model is based on the assumption that if couples knowingly use the female partner's days of fertility for genital intercourse, they are using the method to achieve pregnancy.