The relationship between pregnancy and/or delivery (p&d) and the stage of gastric cancer was studied in 64 female and 57 male patients aged 34 or younger with gastric cancer. Gastric cancer diagnosed within 2 years after p&d (group A, 20 patients) was more progressive (unresectable in 20%) than those of the other young female patients with children (group B, 24 patients; 5%) or without children (group C, 20; 0%), or young male patients (group D, 57; 3%) (P < 0.05). The 5-year survival rate in group A (60.0%) was lower than in group B (83.3%) and group C (85.0%) (P < 0.05 between groups A and B). There were no differences in the duration from the onset of subjective symptoms to diagnosis in the four groups. Out of eight patients who were pregnant after gastrectomy for stomach cancer, one died from recurrence immediately after abortion. These results suggest that pregnancy and/or delivery in young females accelerates the growth of stomach cancer.