This study investigated mothers' perceptions of vaccine-preventable diseases and associated vaccines in terms of perceived susceptibility, severity, benefits and barriers. A purposive sampling strategy was used to choose mothers whose only or youngest child was completely, incompletely (behind the recommended immunisation schedule) or partially (parents chose or advised not to have a specific immunisation) immunised or had no immunisations. Semi-structured interviews found that complete immunisers believed the risk of vaccines was lower than the risk from disease and that the likelihood of contracting many of these diseases was low. Incomplete immunisers perceived vaccines to be less effective in preventing disease and were often confused about which diseases the vaccines would protect against. Non-immunisers were more concerned about unknown, long-term side effects of vaccines than the diseases. Many mothers who did immunise believed that preventing diseases was not always possible and for diseases such as measles, mumps and rubella, it was not always necessary nor desirable. Vaccines were perceived as placing stress on the immune system rather than strengthening it. Important themes relating to barriers to the decision to immunise were a lack of 'balanced', detailed information and poor communication between health providers and parents. The major barrier to timely, age-appropriate immunisations was the occurrence of minor illnesses in the target child or the family. This study found that many mothers were balancing the risks of immunising with the risks of not immunising and this must be taken into account, along with factors such as difficulties in obtaining immunisations.