BACKGROUND: This article reports on formative research into the context and practice of handwashing with soap by new mothers, which can substantially impact child morbidity and mortality. New mothers are an important target group for handwashing interventions: they are considered particularly susceptible to behaviour change and their actions can directly affect a child's health. METHODS: Twenty-seven mothers of infants (including neonates) from urban and rural sub-districts of Serang were recruited and filmed over a period of eight hours. Video footage was used to identify handwashing occasions and to understand the context in which behaviour took place. Each woman was subsequently interviewed. RESULTS: Handwashing with soap was found to be infrequent, typically occurring after eating, cooking and household chores or after cleaning a child's bottom. Handwashing before preparing food or eating was rare. Pre-pregnancy routines were reported to have been disrupted. Advice on child care comes from many sources, particularly the midwife and new child's grandmother. CONCLUSIONS: Developing interventions to change perceptions and practice of handwashing would seed an important behaviour and could save lives. New mothers represent an ideal target group for such an intervention. We suggest that interventions target an increase in handwashing with soap after contact with own and a baby's faecal matter as part of the post-defecation hygiene routines. As the child's grandmother is an authoritative source of information about parenting, interventions focussed on improving newborn care could target grandmothers as well as midwives.