There are two types of neural tube fusion abnormality (NTFA)--craniorachischisis and rachischisis--with a prevalence of 11.2/10,000 in continental Europe. Their prevalence varies widely and is influenced by many factors, including geographical, racial and seasonal components. The transmission of NTFAs is usually polygenic and affected by many factors, and far more rarely of the mendelian monogenic type. The occurrence of NTFAs has been shown to be due to folic acid (FA) deficiency. First suspected in 1965, FA deficiency was proved to be a cause of NTFAs both occurring and recurring in the 1980s. FA deficiency might act by preventing the remethylation of homocysteine to methionine, the latter is known to play an essential role in fusion of the neural tube in animals. Primary prevention of NTFAs therefore requires that FA supplementation be given to women planning a pregnancy and after conception. Two possible ways of preventing FA deficiency can be envisaged, ie. via folic acid supplementation or systemic enrichment of the basic diet. Neither of the two alternatives is sufficient by itself and probably a combination of the two would provide the best means of preventing neural tube defects.