An information programme on measures to prevent passive smoking by children, designed for use during well-child visits, was tested. A total of 443 consecutive families with one or two smoking parents, attending mother and child health centres in Oslo, Norway, were randomly allocated to an intervention group (n = 221) and a control group (n = 222). Eighty families (18%) dropped out during the study period. For the intervention group, the communication between the health visitor and the family was prolonged at one well-child visit with a brief session on smoking, and the parents were given three brochures. The families in the control group received no information on smoking. Changes in practical measures to prevent passive smoking by the children (e.g. no smoking indoors) as well as changes in daily smoking and smoking quantity were assessed by parental reports. We found no significant differences between the groups with respect to change in smoking behaviour.