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Background and purpose: The aim of this study was to compare the proportion of natural killer (NK) and natural killer T (NKT)-like cells in dyspeptic disorders caused by Helicobacter pylori (H. pylori). Materials and methods: In a case-control study, 27 patients with gastric cancer (GC), 25 patients with peptic ulcer (PUD), and 22 patients with non-ulcer dyspepsia (NUD) were enrolled. After endoscopic diagnosis, the proportions of NK cells subsets (CD3-CD56+ and CD3-CD16+), as well as NKT-like cells (CD3+CD56+) in peripheral blood were determined by flow cytometry. The frequency of CD8+ and CD8– NK cell subsets were also determined. H. pylori infection was confirmed in all study subjects using the rapid urease test and histopathology. Results: The frequencies of CD3-CD16+ NK, CD3-CD56+ NK, and CD3+CD56+ NKT-like cells in patients with gastric cancer infected by H. pylori were significantly lower than those in patients with non-ulcer dyspepsia (P < 0.001, P = 0.050, and P = 0.020, respectively). However, these cell populations showed no significant difference between the PUD and NUD groups. Further analysis showed that high numbers of CD8– NK cells in the blood were CD56+ and CD16+ phenotypes. Conclusion: This study showed decreased numbers of NK and NKT-like cell populations in patients with gastric cancer compared to those with non-ulcer dyspepsia. It is thus suggested that these cells may limit the prolonged inflammation by H. pylori, which, in turn, reduce the risk of gastric cancer.