We reviewed the cases of 201 infants hospitalized for "presumed near-miss" sudden infant death syndrome over a recent five-year period to determine the value of routine initial laboratory investigations in establishing diagnosis or altering therapy. A total of 1,278 measurements were performed on 163 patients aged 3 days to 6 months; 276 (22%) of the results were outside the normal limits for age. The most common abnormalities were hyperchloremia (58% of infants tested), hypermagnesemia (43%), hypobicarbonatemia (33%), and hyperkalemia (27%). There were no instances of polycythemia, hypochloremia, hypokalemia, hyperbicarbonatemia, or uremia. Only six (0.5%) of the abnormal values in four patients were useful in diagnosing or treating the apneic episodes. Infants with respiratory symptoms at the time of presentation had a statistically greater incidence of abnormal values than patients who were asymptomatic. Because of the low yield of useful information, we conclude that the use of a routine battery of laboratory investigations in the initial evaluation of infants with presumed near-miss sudden infant death syndrome should be questioned.