Context: After nearly three decades of government-initiated family planning programs, the increase in contraceptive prevalence in Pakistan has been frustratingly slow - from 5% in 1974-1975 to 24% in 1996-1997. At the same time, a significant proportion of women do not wish to have additional children. To understand this contradiction, research is needed to investigate the social, religious and cultural aspects of Pakistani society that may constrain couples' adoption of modern family planning methods. Methods: Interviews were conducted in squatter settlements in Karachi, Pakistan, with Muslim women 30 years old or younger, their husbands and their mothers-in-law to explore factors that influence couples' contraceptive use. Univariate and multivariate regression analyses were conducted to examine the associations between contraceptive use and several variables, including social and demographic characteristics; religious beliefs; communication about family planning among the three family members; women's mobility and decision-making capability; acceptance of information about family planning in the mass media; and exposure to family planning messages from health care workers. Results: Univariate analyses indicate that women who reported using modern contraceptive methods were significantly more likely to be literate (odds ratio, 1.7), to be exposed to an urban environment (1.8) and to have had at least five live births (2.0). According to multivariate analyses, women who were literate, who were of high economic status, whose mother-in-law reported discussing family planning with them and who had received family planning messages from health care workers were 2-3 times as likely to use contraceptives as were other women. In addition, women who said it was appropriate for family planning messages to be delivered through mass media were 50% more likely to use contraceptives. Conclusions: The long-term goals of improving women's education levels and economic status are important for increasing contraceptive prevalence in Pakistan. At the same time, policymakers should initiate short-term interventions, such as engaging religious leaders in family planning programs, encouraging the outreach efforts of community health care workers and targeting mothers-in-law with family planning messages, as these are likely be effective in increasing women's contraceptive use.