Insulin-like growth factor-1 (IGF-1) and its binding proteins (IGFBPs) are produced by many tissues and are present in serum and other biological fluids. Alterations in sera of IGF-1 and 2 and IGFBPs were demonstrated in patients with malignancy, infection and other diseases causing pleural effusion. In this study the IGF-1 and IGFBP-2 content and the specific electrophoretic patterns of IGFBPs in samples of sera and pleural effusions of 25 patients with malignancy, infection and congestive heart failure were investigated. IGF-1 levels in exudative effusions of malignant solid tumors were significantly higher [(mean ± SD), 20.9 ± 7.5 nmol/L, n = 9] than in lymphoma (11.0 ± 5.2 nmol/L, n = 5; p < 0.05), infection (11.4 ± 6.5 nmol/L, n = 6; p < 0.05) and transudative effusion of congestive heart failure (4.3 ± 3.3 nmol/L, n = 5; p < 0.02). IGFBP-2 was markedly increased in effusions of malignant solid tumors (2.14 < 0.82 mg/L, n = 9) compared with exudates of lymphoma, infection and transudates (1.10 ± 0.70, 1.22 ± 0.32 and 0.93 ± 0.52 nmol/L, respectively, p < 0.05). Moreover, in effusion of solid tumors, IGFBP-2 levels were higher than those in corresponding sera, which suggests local production of this binding protein. The demonstration of IGFBP-2 in solid tumor cells by immunohistochemistry further supports this possibility. This work demonstrates the existence of the IGF-1/IGFBP system in pleural fluids from different etiologies and implies possible use of IGF-1 and IGFBP-2 as a potential marker of malignant effusions.