Adult-type hypolactasia is caused by genetic lactase non-persistence. It is the most common cause of lactose intolerance, which results in gastrointestinal symptoms after ingestion of dairy products. Currently, lactose intolerance is investigated by the hydrogen breath test (HBT), which is considered the preferred diagnostic test. Adult-type hypolactasia may also be diagnosed by genotyping. The single nucleotide polymorphism -13910C>T, which is located upstream of the lactase gene (LCT), is tightly associated with lactase persistence. Several other variants, mostly in non-European populations, can also lead to lactase persistence. This study investigated the accuracy of a modified, recently proposed algorithm which includes genotyping for the diagnosis of adult-type hypolactasia in a patient population with unexplained abdominal complaints.In 126 patients with unexplained abdominal symptoms or who were suspected to have adult-type hypolactasia, LCT genotyping by melting curve analysis on a LightCycler was performed. Those patients with CC(-13910) genotype (indicating loss of lactase expression) were directly referred to a dietician for a lactose-free diet. Those identified as CT(-13910) or TT(-13910) genotype underwent a HBT. Those who tested positive for hydrogen were also referred to a dietician for a lactose-free diet. The response to diet modification was recorded.Genotype prevalences were: CC(-13910): 43 (34.1%); CT(-13910): 48 (38.1%); TT(-13910): 33 (26.2%); TG-13915: 2 (1.6%). Eleven of 48 (23%) patients with CT(-13910)-genotype and 1/33 (3%) patients with TT(-13910)-genotype had a positive hydrogen breath test. They all improved after a lactose-free diet. Four of 43 (9%) patients with CC(-13910)-genotype still had symptoms after a lactose-free diet.The results show that lactase-genotype testing can be used as a first step to diagnose lactose intolerance in a patient population with unexplained abdominal complaints. It accurately identifies the group of patients sensitive to lactose, those who need further breath testing and those in whom adult-type hypolactasia can be excluded with high probability without performing a HBT. This algorithm would save hydrogen breath testing in more than 50% of the patients who present with unexplained abdominal symptoms.