The World Health Organization reports that approximately 200 million people are infected with Giardia (G.) lamblia worldwide. Taking into account the emergence of resistance and the high toxicity of conventional drugs, research into new strategies to fight against G. lamblia is increasing. The aim of the study was to assess the antiprotozoal activity of 20-hydroxyecdysone in water sports athletes with giardiosis. A randomized, double-blinded, placebocontrolled clinical study was conducted. Seventy-six athletes with G. lamblia infection participated in the study and were divided into 20-hydroxyecdysone, metronidazole and placebo groups. Clinical, parasitological, haematological and biochemical analyses were performed. Positive results for antiprotozoal therapy were revealed in the 20-hydroxyecdysone and metronidazole groups. After therapy, elimination of G. lamblia was observed in 100.0% of the athletes included in the 20-hydroxyecdysone group. However, G. lamblia was resistant to metronidazole in 4.0% of athletes included in the metronidazole group. A positive clinical response to the therapy occurred in the 20-hydroxyecdysone and metronidazole groups. Our study reveals high antiprotozoal activity of 20-hydroxyecdysone against G. lamblia. Further clinical studies are necessary to evaluate the antiprotozoal efficacy of 20-hydroxyecdysone.