A prospective six-year study (1975-1980) of 273 patients, monitored in the use of natural family planning (NFP), has shown that those with previously irregular menstrual cycles are disadvantaged in the subsequent use of the ovulation method after pregnancy in that they have fewer recognizable safe days and/or are more likely to have unplanned pregnancies than women who had regular cycles prior to pregnancy. Eight patients, seven nursing and one non-nursing mother, conceived during postpartum amenorrhoea. The reliability of memory in the recall of previous menstrual histories is discussed in relation to the results of an international study of menstrual cycles by the World Health Organization (WHO 1983). The relevance of prediction of menstrual events in natural family planning is also considered.