Helicobacter pylori infection is associated with abnormalities in serum gastrin concentration, antral gastrin and somatostatin content, and D-cell density in adults. We have studied the effects of H pylori infection in children. We studied 13 children positive for H pylori and 7 negative children. The median antral somatostatin content was significantly lower in the positive than in the negative group (0.69 [range 0.35-0.91] vs 1.31 [0.73-1.67] ng/mg, p = 0.007). Both antral and serum gastrin concentrations were significantly higher in the positive group (30.1 [15.3-83.6] vs 14.8 [13.8-28.8] ng/mg, p = 0.008; and 89.9 [59.4-313.2] vs 29.5 [13.9-71.1] pg/mL, p = 0.006). Treatment to eradicate H pylori was successful in 11 of the 13 positive patients. With eradication antral somatostatin increased to within the normal range (by a median of 0.41 [0.21-0.86] ng/mg to 1.10 [0.81-1.55] ng/mg, p = 0.016). Serum and antral gastrin decreased (by 37.1 [5.5-265.2] pg/mL to 52.8 [21.4-267.5] ng/mg, p = 0.001; and by 8.0 [2.0-47.2] ng/mg to 22.1 [10.9-37.5] ng/mg, p = 0.001). Eradication of H pylori also significantly increased antral D-cell density (8 [5-22] to 15 [9-22] cells per mm, p = 0.031) and decreased G-cell density (138 [89-161] to 88 [33-121] cells per mm, p = 0.016). The hypergastrinaemia in children positive for H pylori may be due to a deficiency of antral somatostatin, which inhibits gastrin synthesis and release.