To evaluate the importance of the raised levels of growth hormone that characterize poor diabetic control, we gave growth hormone for 21 to 45 hours in the form of hourly 100-micrograms pulses to 14 diabetics being treated by insulin pump. Insulin-pump settings and meals were kept constant. Mean 24-hour levels of growth hormone (+/- S.E.M.) rose from 8 +/- 1 to 16 +/- 2 ng per milliliter--values identical to those observed in 12 other patients with poorly controlled diabetes (17 +/- 3 ng per milliliter). Plasma glucose concentrations doubled within 8 to 10 hours and remained elevated until growth hormone was discontinued (fasting glucose level rose from 86 +/- 11 to 204 +/- 17 mg per deciliter at 18 hours and to 240 +/- 20 mg per deciliter at 42 hours). The hyperglycemia was due mainly to a marked stimulation of hepatic glucose production that occurred without changes in levels of free insulin or glucagon. Levels of circulating free fatty acids, ketones, and branched-chain amino acids were also increased. The moderate elevations in growth hormone levels that occur in poorly controlled diabetes can themselves reproduce the whole spectrum of abnormal metabolic fuel concentrations that are associated with poor diabetic control, despite optimized insulin treatment. Thus, hypersecretion of growth hormone may be the cause as much as the consequence of poor diabetic control.