UNLABELLED Patients with cystic fibrosis (CF) are underweight and growth retarded. This study tested the link between serum insulin-like growth factor-I (IGF-I) and insulin-like growth factor-binding protein-3 (IGFBP-3) levels and body height, nutritional status, pulmonary function tests and activity of inflammation in 92 subjects with CF (age 2.1-18.8 y). It also analysed the effect of short-term antibiotic treatment and hyperalimentation on IGF-I and IGFBP-3 levels in 33 subjects (age 3.6-33.7y) on 41 occasions. Both IGF-I (-1.19 +/- 0.17 SD) and IGFBP-3 levels (-0.66 +/- 0.12 SD; both p < 0.0001 vs 0) were decreased in cross-sectional measurements. Their standardized values were inversely proportional to age (IGF-I: r = -0.23, p = 0.03; IGFBP-3: r = -0.29, p = 0.005) and positively correlated with SDS of height (IGF-I: r = 0.40, p < 0.0001; IGFBP-3: r = 0.36, p = 0.0005) and of mid-arm circumference (IGF-I: r = 0.39, p = 0.0001; IGFBP-3: r = 0.38, p = 0.0002), and with pulmonary function tests. After a short-term course of intensive antibiotic therapy and hyperalimentation, IGF-I normalized (from -0.66 +/- 0.20 to 0.00 +/- 0.25 SD; p < 0.0001) and IGFBP-3 increased (from -0.78 +/- 0.15 to -0.53 +/- 0.16 SD; p = 0.002). IGFBP-3 correlated inversely with erythrocyte sedimentation rate (r = -0.40, p = 0.01). CONCLUSION The levels of IGF-I and IGFBP-3 are markedly decreased in patients with CF and tend to normalise after a short course of antibiotic treatment and hyperalimentation.