We compared clinical presentation and outcome of 225 children with severe pneumonia between those with and those without diarrhea. Having diarrhea was associated with metabolic acidosis (P < 0.001) and hypocalcemia (P = 0.002) on presentation, and with a fatal outcome: 20/113 (20%) died with diarrhea versus 3/112 (3%) without diarrhea. Diarrhea is an important comorbidity in Bangladeshi children with severe pneumonia requiring attention for improved case management.