Urinary growth hormone (uGH) excretion was evaluated in 96 type-1 insulin-dependent diabetic patients and 37 age-matched healthy subjects. The growth hormone concentration was measured by a solid-phase immunoradiometric assay on 3 consecutive overnight urine collections. uGH excretion was comparable between diabetic patients and healthy subjects: 10.9 (0.1-34.8) vs. 9.1 (2.6-34.5) pg/min. In both groups uGH excretion was lower in prepubertal than in pubescent or pubertal individuals (diabetic patients, H = 29.7, p = 0.001; healthy subjects, H = 10.4, p = 0.006). In diabetic patients uGH excretion was related to beta 2-microglobulin excretion (r = 0.308; p = 0.005) and to urinary albumin excretion (r = 0.230; p = 0.02) but it was independent of HbA1c and overnight glycemic values. The coefficient of variation of uGH excretion was higher in diabetic patients with respect to healthy subjects: 50 (3-141) vs. 28(3-100)% (p = 0.002). Among diabetic patients it was greater in prepubertal than in pubescent or pubertal patients (H = 13.7; p = 0.002); in contrast, it was independent of pubertal stage in healthy individuals (H = 2.4; NS). The coefficient of variation of uGH was not related to HbA1c, the duration of diabetes, the coefficient of variation of urinary albumin excretion and the coefficient of variation of beta 2-microglobulin excretion. In conclusion uGH excretion is comparable among diabetic patients and healthy subjects, but its day-to-day fluctuation is greater in the former than in the latter group. Renal function but not metabolic control can influence uGH excretion. The day-to-day fluctuation in uGH excretion is independent of metabolic control and renal function.