This prospective study evaluated the frequency, clinical characteristics and causes of hyponatremia (serum sodium < 130 mEq/L) in 727 children upto 12 years of age, who were brought for emergency care, and needed hospitalization. Hyponatremia was found in 29.8% and was more frequent in summer (36%; 123/341) than in winter (24%; 94/386) (p < 0.001). Acute lower respiratory infections (pneumonia) and acute diarrhea each accounted for 20% cases of hyponatremia; others were accounted for by meningitis/encephalitis (12%) septicemia (8%), and renal, heart and liver diseases (6-7% each). Clinical evaluation and concurrent plasma and urinary osmolality and urine sodium suggested that hyponatremia associated with pneumonia, meningitis/encephalitis, septicemia, seizures and miscellaneous diseases was of hypotonic-euvolemic (dilutional) type in more than 80% patients while in all children with acute diarrhea it was of hypovolemic type. The study has shown that hyponatremia occurs frequently in sick children requiring emergency care, especially in summer months, and should receive appropriate attention in the management plan.