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PURPOSE OF THE STUDY The aim of the study was to evaluate the safety and efficacy of the application of platelet- and leukocyte-rich plasma (L-PRP) in the treatment of lateral humeral epicondyle (ERH) as compared to the administration of corticosteroid therapy. MATERIAL AND METHODS It was a prospective, double-blind and randomized clinical trial. It included a total of 25 cases of ERH in 23 patients aged 18 to 60 years. They were divided into two arms: L-PRP arm with 10 cases and corticosteroid arm with 15 cases. The therapy of L-PRP group: first application: 3 ml of native L-PRP without activation, second application: after 6 weeks, 3 ml of thawed L-PRP. Therapy of the corticosteroid arm patients: first and second application (both after 6 weeks) of 3.5 mg of Betamethasonum. The follow-up period was 12 months. The safety evaluation was carried out based on the number of adverse events. The efficacy was evaluated using the DASH score and the Visual Analogue Scale (VAS). The time series outcomes were analysed statistically by percentile graphs and repeated measures ANOVA. RESULTS The ascertained values of blood elements in the received L-PRP as against the original blood were the following: platelets around 400%, white blood cells around 200%, red blood cells around 30%. The differences in individual subpopulations of white blood cells: neutrophils 74%, lymphocytes 424%, monocytes 385%, eosinophils 43%, basophils 337%. The product was classified as L-PRP group based on the POSEIDO classification. Safety evaluation: both the groups reported no adverse and also no serious adverse events. One patient withdrew from the corticosteroid group on the grounds of a subjective feeling of treatment failure. Efficacy evaluation: for both the general DASH score and Work Module a statistically better effect of L-PRP was proven in the first 6 months at p < 0.05. For the general DASH score, the L-PRP group reported a decrease of difficulties from approximately 50% to 20% on DASH score and in the corticosteroid group the difficulties returned to more or less the initial value. In the Work Module, the L-PRP group showed a decline in difficulties from approximately 46% to 18% and the corticosteroid group from approximately 43% only to 38%. The Sports and Music Module was evaluated by percentile graphs only. The VAS assessment proved that an effect was achieved only after the administration of the first dose of L-PRP and corticosteroid, with a more significant effect reported in L-PRP. The overall statistical evaluation using the VAS showed no difference between these two therapies at the level of significance of p<0.05 (p = 0.08). DISCUSSION The product used by us was classified as a L-PRP subcategory. The application of the native and subsequently frozen L-PRP is rare as compared to the corticosteroid therapy. CONCLUSIONS According to our results, the application of L-PRP is suitable to treat lateral humeral epicondyle. The benefits can be foreseen in the form of a better functional score, whereas the analgesic effect equals the effect of corticosteroid. A slightly additive effect was seen in the thawed second dose. Key words: platelet- and leucocyte-rich plasma, epicondylitis radialis humeri.