• We tested the hypothesis that integrated concentration of human growth hormone (ICGH) during the 12-hour nighttime (8PM to 8 AM) period is closely correlated with the 24-hour integrated concentration and would be as useful for diagnostic purposes. We determined hourly ICGH during 24 hours in 67 children and calculated the 12- and 24-hour values. Nineteen children had normal stature, 13 had classic growth hormone deficiency, four had growth hormone neurosecretory dysfunction, 23 had short stature but no hormonal abnormality could be diagnosed, and eight had miscellaneous disorders. There was a close correlation between the 12- and the 24-hour ICGH. For diagnostic purposes, 12-hour nighttime ICGH is as useful as 24-hour ICGH. Use of the shorter test for determination of physiologic growth hormone secretion will save the patients time, expense, and blood volume.