We compared the lactose hydrogen breath (lactose HBT) and lactose tolerance tests (LTT) in their ability to diagnose lactose malabsorption (LM).Fasting and post-lactose (50 g) breath hydrogen and blood sugar were tested in patients with irritable bowel syndrome (IBS). Persistent rise in breath hydrogen by 20 ppm and failure of blood sugar to rise by >20 mg/dL above basal level, were considered positive lactose HBT and LTT, respectively. Symptoms of diarrhoea, bloating, abdominal pain and flatulence were noted.Of 203 patients, 11 demonstrated high basal breath hydrogen and hence, 192 (age 37+/-14 years, 134 male) were included in the study. 125 (65%) and 137 (71%) were lactose HBT and LTT positive, respectively. 102/125 lactose HBT positive patients were LTT positive and 35/67 lactose HBT negative patients were LTT positive. 62/192 (32%) developed symptoms following lactose ingestion, which tended to be more in the LTT positive (49/137, 36% vs. 13/55, 24% p=0.07) but not in the lactose HBT positive patients (44/125, 35% vs. 18/67, 27% p=0.2). Peak breath hydrogen was higher (38+/-37 vs. 66+/-43; p<0.01) in LTT positive than negative patients. Peak level of breath hydrogen inversely correlated (58+/-43 vs. 10+/-23; p<0.001) with change in blood glucose following lactose ingestion.Positive LTT is associated with a higher breath hydrogen score than negative LTT. There was a trend towards more frequent symptom development following lactose load in LTT positive but not in lactose HBT positive patients. LTT is an easy and efficient test for diagnosis of LM.