The uptake rate for primary immunisation against diphtheria in 1987 in the east part of Riverside District Health Authority was 61% for children aged two. A retrospective survey was undertaken of the immunisation records of all children aged two recorded on the Child Health System as not fully immunised: 46% had been immunised; 18.1% had moved out of the district; 15.6% of the children could not be traced; in 13.6% of records, there was no reference to immunisation; and only 2.5% of children were definitely not immunised. Large numbers of omissions were found in the data held on the Child Health System. The reasons for this are discussed and recommendations made for improving the quality of the data held.