Aim. To identify poor prognostic factors for perianal infection (PI) in patients with hemoblastosis and to define an effective tactic for preventive and therapeutic measures. Subjects and methods. The prospective study enrolled 72 patients (37 men and 35 women; mean age, 47 years) with hemoblastosis that was complicated by the development of one of the following forms of PI: abscess, infiltrate, multiple ulcers. Different clinical and laboratory characteristics of the patients were examined to identify risk factors for PI. The species-specific concordance of microorganisms isolated from the anus and blood in the development of PI was assessed to record the latter as a source of sepsis. Treatment policy was defined according to the clinical form of PI. Results. Acute myeloid leukemias and lymphomas were the most common background diseases in 30 (41.7%) and 22 (30.6%) patients, respectively. During induction chemotherapy cycles, perianal tissue infection occurred twice more frequently (66%) than totally at the onset of hemoblastosis (13%) and after achievement of remission (during consolidation and maintenance therapy) (21%; Fisher’s exact test; p=0.01). PI in agranulocytosis was more than twice as common as in its absence: 69.4% vs 30.6% (p=0.01) and was responsible for sepsis in 9 (18%) of 50 patients. The main source of perianal tissue infection in patients with granulocytopenia was anal fissures and fistulas and ulcers of the anal canal: 44 (88%) cases of the 50 cases. In PI as an abscess, the average white blood cell count was 5 times higher (p=0.01) than that in PI as an infiltrate (or multiple ulcers): 6.6·109/l and 1.2·109 g/l. Abscess formation was observed in 16 (22.2%) patients and an indication for surgical drain. The inflammatory infiltrate was found to develop in 48 (66.7%) patients; multiple ulcers were seen in 8 (11.1%); in this group, parenteral antimicrobial therapy proved to be effective in 36 (78%) patients. 29 patients were operated on for anal fissures and fistulas at intercycle intervals. After continuing CT, PI recurrences were observed in 4 (9.1%) patients. In the operated versus medically treated patients, the risk of complications associated with abnormalities in the perianal area during continued CT was 5 times statistically significantly lower (odds ratio=0.2; 95% confidence interval 0.1 to 0.5; p=0.04; Cochran-Mantel test). Conclusion. Induction CT cycles, the status of granulocytopenia, and the presence of infection sources in the anal canal as an anal fissure, skin ulcerations, or a fistula should be considered as independent statistically significant prognostic risk factors for PI. The number of granulocytes determines the form of inflammation, the course of infection, and the chance of developing sepsis. The effective prevention encompassing surgical treatment for anal canal diseases reduces the risk of septic complications and the number of paraproctitis recurrences, contributing to the implementation of a planned CT program in patients with hemoblastosis.