We report on a 5 year experience in providing presymptomatic and prenatal molecular diagnostic services for myotonic dystrophy, using closely linked markers, representing 235 completed results in 161 families. Only 10 analyses (4.3%) proved uninformative, but a further 5 requests (1.9%) could not be reported because of uncertainty in clinical status. Seven of 81 (8.6%) patients considered to be at low risk on clinical grounds were found to be at high risk of carrying the gene. The importance of interpreting molecular results in conjunction with clinical findings is emphasised by the illustrative examples provided. Careful clinical examination and appropriate investigation remain a cornerstone of diagnosis in myotonic dystrophy and are crucial if errors in assigning genotype status by molecular means are to be minimised.