This is an observational quantitative and analytical study aimed at verifying the knowledge, acceptability and use of natural family planning (NFP) by patients in a university hospital from July to November, 2008. The data were collected using a structured questionnaire and analyzed with the softwares Excel and Statistica 8.0. Of the 113s women interviewed, 70 (62%) accepted the method and 1 (0.9%) used it routinely. Acceptance was higher among those who wished to become pregnant in the future compared to those who did not wish it. Acceptability was statistically significant (p = 0.0147) among the 28 (80%) non-contraceptive users compared to 42 (53.8%) who used some contraceptive method. Factors such as age, education, number of living children and religion were not statiscally associated with the acceptability of NFP. The Billings ovulation method has an adequate acceptability, but has a low actual use because of the lack of information by health professionals of its real effectiveness and applicability.