The object of the study was to evaluate R. Klimek's new simple clinical method of computing newborn's maturity index (K) without having to use the computer-aided method or understand the basics of the quantum mechanics or theory of relativity, which gave rise to the modern definition of maturity. The study was based on the material of 1000 successive newborns delivered in the Clinic of Gynecology and Obstetrics of the Jagiellonian University. Their maturity was determined routinely in BK scale, and confirmed by means of computer-aided method of birth term prognosis. Posture, angle of the elbow, pulling an elbow to the middle line of the body, breast appearance, presence of lanugo and plantar creases were evaluated by means of K index with points from 0 to 2. Newborns' maturity index K was 9.2 +/- 1.9 points (range 2-12). 2.6% newborns were immature and one third of them (0.8%) were born with the gestational age < 37 weeks, 28 of which (77%) had correct K index > or = 6 points. There is a high, statistically significant correlation between maturity assessed in Ballard-Klimek scale and K index (r = 0.61, t = 24.78).