Hyperkalemia is common soon after birth in extremely premature infants and often occurs in the absence of oliguria or renal failure. Our objective was to examine the early biochemical indicators, clinical risk factors, and incidence of early-onset nonoliguric hyperkalemia in a cohort of extremely premature infants. We studied clinical and biochemical data from 154 consecutive premature infants < or = 28 weeks of gestation admitted to a tertiary neonatal unit over a 3-year period. Hyperkalemia (> 7 mmol/L) was found in 33 (21%) of patients during the first 4 days of life, with peak potassium levels occurring at 3 days of age. Hyperkalemic patients had similar birthweight, gestation, creatinine and sodium levels as normokalemic infants, but they had higher phosphate and urea levels, with lower calcium levels. The combination of a high phosphate (> 2.0 mmol/L) and a high potassium (> 5.6 mmol/L) within 6 hours of birth predicted later development of hyperkalemia, with a positive likelihood ratio of 8.3 (post-test probability, 70%). Potassium level of < 5.6 mmol/L at birth indicates subsequent hyperkalemia is unlikely (likelihood ratio, 0.25; post-test probability, 4%). Mildly elevated potassium and phosphate levels within 6 hours after birth may be useful in predicting the development of early-onset hyperkalemia.