This paper describes the evaluation of a new method of natural family planning (NFP) in Liberia. The Modified Mucus Method (MMM) was developed to address the need for a simple method of charting for poor and illiterate women. The acceptance, use, and cost-effectiveness of the MMM were compared with standard NFP methods, the sympto-thermal and ovulation method (ST/OM), used in the same population. The personal discontinuation rate of MMM users was 27.3 per 100 women per year compared with 3.2 among ST/OM users. Unplanned pregnancy rates were low for both MMM and ST/OM, 6.6 and 1.5 respectively. The cost per couple year protection (CYP) for MMM was $55.80 and for ST/OM $56.10. There were differences in characteristics between MMM and ST/OM clients. The MMM clients were more likely to have attended school and to have used a family planning method previously, and were less likely to be housewives. We conclude that the MMM in Liberia was provided to an inappropriate sample of women, educated and middle-class rather than poor and illiterate. The MMM users were dissatisfied and discontinued at the rate of 44 per 100 women entering per year. This is an unfair evaluation of the MMM because of the unsuitable study population. It is our opinion that the MMM needs more study to become part of the inventory of birth spacing methods.