BACKGROUND: Drug therapy for people with AIDS is a humanitarian priority but prevention of HIV infection remains essential. Focusing on young single African women, we aimed to assess trends in a set of behaviours-sexual abstinence, contraceptive use, and condom use-that are known to affect the rates of HIV transmission. METHODS: We did a secondary analysis of public-access data sets in 18 African countries (132,800 women), and calculated changes in a set of behavioural indicators over time. We standardised these trends from nationally representative surveys to adjust for within-country changes in age, education, and type of residential location. FINDINGS: Between about 1993 and 2001, the percentage of women reporting no sexual experience changed little. During the same period, the percentage of sexually experienced women who reported no sexual intercourse in the previous 3 months (secondary abstinence) rose significantly in seven of 18 countries and the median for all 18 countries increased from 43.8% to 49.2%. Use of condoms for pregnancy prevention rose significantly in 13 of 18 countries and the median proportion increased from 5.3% to 18.8%. The median rate of annual increase of condom use was 1.41 percentage points (95% CI 1.12-2.25). In the 13 countries with available data, condom use at most recent coitus rose from a median of 19.3% to 28.4%. Over half (58.5%) of condom users were motivated, at least in part, by a wish to avoid pregnancy. INTERPRETATION: Condom promotion campaigns in sub-Saharan Africa have affected the behaviour of young single women; the pace of change has matched the rise in contraceptive use by married couples in developing countries over recent decades. Thus continuing efforts to promote condom use with emphasis on pregnancy prevention are justified.