Abdominal pain in children is one of the most common reasons for consultations in general practice. Diagnostics of abdominal pain in children is particularly difficult. One of the causes of this disorder is lactose intolerance due to lactase deficiency. The aim of this study was to assess the frequency of lactose malabsorption and lactose intolerance in children with abdominal pain, as well as to investigate the time-lag between onset of symptoms and diagnosis. The study involved 170 children with abdominal pain, aged 2-18 years. Some patients suffered from other gastrointestinal symptoms, such as vomiting, nausea, abdominal distension or diarrhea. Children were divided into 2 groups. The first group comprised 121 children aged 2-7 years (mean age 3.8 ± 1.4 yrs), 60 girls and 61 boys, and the second group consisted of 49 subjects aged 8-18 years (mean age 12.9 ± 3.5 yrs), 24 girls and 25 boys. Hydrogen breath test (HBT) after oral ingestion of lactose was performed in all children. All subjects remained under observation for 24 hours. The development of adverse gastrointestinal symptoms in response to the ingestion of a given amount of lactose was monitored. The time interval from the first symptoms of lactose intolerance to diagnosis was also estimated. In the first group, positive results of HBT test confirming lactose malabsorption were observed in 16 children (13%). Clinical symptoms after oral ingestion of lactose occurred in 13 children (10.7%), 6 (26.7%) children with positive and 7 (5.7%) with negative results of this test. In the second group, the HBT results indicating lactose malabsorption were obtained in 17 children (34%). Clinical symptoms after oral lactose ingestion developed in 10 (20.4%) of the children, 6 (28%) with positive and 4 (8.1%) with negative result of the test. In the first group of children, the time-lag between the first symptoms and proper diagnosis of lactose intolerance was mean 7 ± 2 months, while in the second group it was 14 ±4 months. Among children aged 2-7 years, the frequency of lactose malabsorption and lactose intolerance were 13% and 10.7%, respectively. Among children aged 8-18 years the frequency of lactose malabsorption and lactose intolerance were 34% and 20.4%, respectively. The frequency of lactose intolerance shown by this study, as well as the relatively long period of time required to make correct diagnosis, indicate that this disorder should be taken into account more often when diagnosing abdominal pain in children.