In the present study we evaluated somatomedin-C (Sm-C) plasma levels in diabetic patients, with and without retinopathy. One hundred and thirty four diabetic patients (65 type I and 69 type II) and 90 controls, strictly matched for age and sex, were enrolled in the study. Ophthalmoscopy and fluorescein angiography allowed to distinguish: 49 patients without retinopathy, 45 patients with background retinopathy, and 40 with proliferative retinopathy. Growth hormone (GH) and Sm-C plasma levels were measured using a pool of 20-24 blood samples over 24h. Sm-C levels in type I (0.62 +/- 0.11 U/ml) and type II (0.56 +/- 0.09 U/ml) patients were significantly decreased (p less than 0.01) when compared to controls (0.89 +/- 0.30 U/ml). The mean daily secretion of GH was significantly (p less than 0.01) greater in diabetic patients (7.8 +/- 2.6 ng/ml) than in controls (4.1 +/- 1.5 ng/ml), but no correlation was found between Sm-C and GH (r = 0.15; p = n.s.). Our findings did not show any correlation between Sm-C plasma levels and either the existence of retinopathy, regardless of the degree of microvascular damage, or duration of the disease, or degree of metabolic control, as evaluated by HbA1c.