The paramyxoviruses respiratory syncytial virus (RSV) and parainfluenza virus type 3 (PIV3) are the two most common agents of severe lower respiratory tract disease in infants and children throughout the world. RSV causes yearly epidemics of bronchiolitis and pneumonia in infants and young children, while PIV3 is a common cause of bronchiolitis, pneumonia and croup. Together these two agents account for up to 30% of all hospitalizations of infants and young children for respiratory tract disease. A licensed vaccine is not currently available for either of these viruses. Development of vaccines against diseases caused by RSV and PIV3 is one of the priorities of the Global Programme for Vaccines (GPV). On 27 March 1994, GPV sponsored a workshop in Nyon, Switzerland, to review the status of vaccine development for these pathogens and to explore new methods of immunization that might be applied to the prevention of diseases caused by RSV and PIV. Furthermore, the World Health Organization (WHO) wished to assess progress in the development of methodologies to rescue infectious virus from cDNA clones of RSV and PIV3. This technology, when developed, will be extremely valuable in developing new vaccine candidates and in unravelling the genetic basis of attenuation of existing vaccines. This paper summarizes the findings presented at this one-day meeting.