PIP: 4000 pregnant women were specifically asked about the method of contraception they last used and whether their pregnancy was a consequence of a failure of contraception. Social, economic, and religious variables were examined to assess the extent to which these were associated with differing rates of contraceptive failure. As part of a longtitudinal study of the outcomes of pregnancy, all women seeking antenatal care at a large public hospital in Brisbane (Australia) were enrolled at their 1st visit. The direct method of standardization was used to examine the contraceptive failure rate for the demographic variables of interest. This allowed adjustment for any effect of associated variables by calculating a weighted average of the specific rates within each category. Variation in the standardized rates of contraceptive failure was examined by a chi-squared test and, where appropriate, supplementary stepwise chi-squared tests were used to detect an upward or downward trend in rates for an ordinal demographic variable. While the rate of unwanted/unplanned pregnancies differed according to the criterion chosen, the variation was between 22% who believed their pregnancy was attributable to a contraceptive failure and 58% who stated that they did not plan their pregnancy or were unsure about whether it was planned or not. Altogether 29% of women acknowledged the possibility or probability of contraceptive failure. Single women were about twice as likely as married women to report contraceptive failure. These differences were independent of age differences between the marital status group. Single women and those "living together" reported the highest failure rates regardless of the most recent method of contraception used. The standardized rate at which a particular method was reported to fail did not relate significantly to the mother's educational or occupational status. More educated women had higher contraceptive failure rates because they were more frequent users of the "rhythm" method and this method fails about half the time. Lower income women reported higher rates of contraceptive failure because they tended to be less successful users of oral contraceptives (OCs. Reported contraceptive failure rates appeared relatively consistent for the method of contraception reported. Women who were more frequent church attenders reported somewhat higher rates of contraceptive failure.