The use of techniques for the production of prometaphase chromosomes leading to increased resolution of banding patterns has resulted in problems as well as advantages in their routine use. A simple technique utilising the folate antagonist methotrexate has been in use in this regional diagnostic cytogenetics centre for three years, and a number of chromosome abnormalities have been detected in cases previously reported as normal following G banding studies. With reference to this, the routine analysis of prometaphase-type chromosomes is discussed and assessed.