The aim of the study was to investigate the reproduction efficiency of a medicinal leech Hirudo verbana Carena, 1820. Totally 25 gravid medicinal leeches weighted 5.70 +/- 3.22 g were used. The leeches deposited 3.20 +/- 1.87 cocoons by losing 23.46 +/- 5.42% in body weight in every cocooning at 9.67 +/- 3.68 days intervals. The cocoons, having a weight 0.93 +/- 0.38 g, diameter 13.81 +/- 2.26 mm and length 21.44 +/- 3.81 mm, had 12.29 +/- 5.14 offspring weighted 29.11 +/- 13.62 mg. It was determined the negative correlation between the adult leeches weight and the weight loss percentage after each cocoon deposition, between the adult leeches weight and the cocoon deposition frequency, and between the number of offspring and the mean weight of them. The positive correlation was found between the adult leech weights and the number of the laid cocoons, between the adult leech weight and the laid cocoon weight, and between the cocoon weight and the number of offspring. H. verbena shows strong reproduction effort, it can reproduce even loosing 70% their initial weights. Reproduction success of the medicinal leech is strictly correlated with the mature individual weight. Therefore, heavier individuals should be chosen and used in artificial production procedures. Collection of heavier specimens from natural habitats must be prohibited to conserve natural populations