Studies on UMPK polymorphism were carried out in a sample of the Polish population numbering 462 subjects. The occurrence of three phenotypes was confirmed--UMPK 1, UMPK 2-1 and UMPK 2. From distribution of phenotypes, frequencies of determining genes were calculated with the following results: UMPK1-0.9762, UMPK2-0.0238, The usefulness of this system in paternity cases is 1.69%.