BACKGROUND Infants less than 1500 grams at birth have been demonstrated to be particularly prone to development of low levels of serum sodium often leading to increased early neonatal morbidity and mortality. No local study has been done to quantify this problem among sick newborns. Studies elsewhere demonstrate a high incidence of hyponatraemia among such preterms. OBJECTIVE To evaluate the influence of infant early neonatal morbidity on serum sodium levels and justify regular monitoring and supplementation. DESIGN Comparative cohort study. SETTING Newborn Unit, Kenyatta National Hospital, Nairobi. SUBJECTS Fifty six very low birth weight (1000-1500 grams at birth) infants during their first week of life. Half of them were designated as cases in view of having various early neonatal illnesses. The remaining 28 being clinically stable were taken as controls. These two groups had comparable birthweights, sex distribution and gestational ages. Their sodium intakes were also similar during the first week of life. RESULTS The sick infants (cases) had persistently low serum sodium (mean of 120 mmols/L) throughout the first week while among the healthy infants (controls) a sequential increase from 127 to 133 mmol/l, (mean values) was observed during the same period. The difference registered on day seven (133 versus 120) was statistically significant (p = 0.02). Using a cut off point of 130 mmol/L to define hyponatraemia the proportion of infants with hyponatraemia, which was similar at the beginning became higher among the cases for the rest of the week with the largest disparity observed on the seventh day (75% versus 23%, p = 0.007). Urinary sodium losses as measured by Fractional Sodium Excretion were also initially similar between the two groups but later became higher among the cases (4.96 versus 3.5 p=0.08). CONCLUSION Very low birth weight infants who are ill have lower serum sodium and are more likely to develop significant hyponatraemia than their healthy counterparts during the first week of life. Standard care of these sick infants must therefore routinely include regular monitoring of serum sodium and its correction if found to be low.