An oral folate absorption test was performed in 100 consecutive patients with Crohn's disease (CD) and 20 healthy individuals to investigate the frequency of abnormal folate absorption in regard to the site of the disease and to investigate the possibility of defining a subgroup of patients requiring parenteral folate supplementation. The described oral folate absorption test can be performed quickly on an outpatient basis and is capable of distinguishing patients with altered folate absorption from those with normal folate absorption. In 25 patients, abnormal folate absorption was detected. 16 patients showed impaired folate absorption as indicated by a marked but insufficient increase in serum folate levels after oral folate intake, whereas no increase of the serum folate levels was detected in the remaining 9 patients. Abnormal folate absorption was not correlated with disease extent or activity. In patients with only impaired folate absorption, it might be sufficient to increase dietary intake of folates. In the remaining patients with no measurable increase of serum folate levels after oral folate intake, i.e. about 10% of all patients with CD, parenteral folate supplementation could be considered.