More than 3000 direct observations of 5900 mooses were used to investigate the sex structure and growth of the population living in the Kampinoski National Park (KNP, central Poland). The study used data collected by the park service since 1998. To confirm the hypothesis that the random observations will accurately describe the population structure the experiment was performed with a theoretical population. It was a collection of 102 numbers (1−39 for females, 40−81 for males, and 82−102 for calves). Sex structure of this population was 1:1 and the growth rate was 0.6 calf/female. To provide the randomness all ‘animals' were mixed and renumbered. Each time we drew 4 numbers (like 4 random moose observed). The drawn numbers were grouped into: males, females and calves. Results of the consecutive draws were accumulated, while sex ratio and growth rate of this hypothetical population were constantly recalculated. Changes in ratios with increasing abundance of ‘recorded' moose were observed. It was assumed that the accuracy of estimates of population structure will increase with the number of observed animals and ultimately the results of the draw will show real sex structure and growth of analyzed population. The draw was repeated three times. Each time the results confirmed the hypothesis. A similar procedure was performed for empirical data from KNP. The documented direct observation of moose throughout the year provides a reliable assessment of the sex structure and population growth. At a constant, daily assessment of gender structure requires longer observation time than evaluation of growth. Application of these traits based on a small empirical material and/or short follow−up time may result in significant errors. The sex structure of moose in KNP is at the level of about 1:2, while growth at 0.8 calf/female. The values of indicators describing these characteristics are higher than previously adopted for the purposes of hunting of moose use in Poland. It would clarify the methodology of recording observations – a clear definition of calf age (0−1 years) and a separate listing of moose treated as an independent one− −year age group. It is also strict principle of not combining several separate observations (e.g. summary of the daily observations) as one independent observation. The method of accumulation of direct observation to assess the gender structure and growth should be tested in other deer species.