Fractal dimension of pericellular membrane of monocytes was evaluated in diabetic patients and in control subjects. Monocytes were collected from normal healthy volunteers (n = 6) and from diabetic (type 1 and type 2) patients (n = 9). Monocytes from healthy volunteers were also stimulated in vitro with the ionophore A23187 or with the oligopeptide FMLP. Monocytes, obtained by Ficoll-Hypaque, were examined with a Philips 300 transmission electron microscope. The cell contour was extracted, resized to a standard dimension and converted to a single pixel outline. Box-counting method was then applied to determine the fractal dimension. Fractal dimensions of monocytes appeared statistically increased in diabetic patients (type 1 and type 2), compared with sex- and age-matched controls (p < 0.01, p < 0.01). The mechanism underlying the observed increased complexity of pericellular membrane may be explained by the in vivo activation of the circulating monocyte in diabetes. In effect, fractal analysis of stimulated in vitro monocytes showed a significant increase of complexity of pericellular membrane, compared with their controls (p < 0.001). Our approach was able to assess and quantitatively evaluate in diabetic patients morphological modifications of the monocyte linked to its activation, offering new parameters useful to follow the effects of therapeutical procedures.