The aim of this study was to determine the degree to which general body fatness variation, presented by body mass index (BMI), the sum of the three skinfold thicknesses (TST) (triceps, subscapular, abdominal) and percentage of body fat (%FAT), can be explained by socioeconomic status (SES) and lifestyle. The cross-sectional, population-based survey was of 259 healthy working males aged 20–30 from the city of Cracow, Poland. Objective anthropometric measurements, bioelectrical impedance analysis, the results of motor fitness tests and social and lifestyle data from a questionnaire were analysed. The independent variables were: age, socioeconomic status (birthplace, place of residence until the age of 14, social class, educational level and the type of work done) and lifestyle elements (smoking habits, dietary habits, family obesity resemblance, sport activity in the past, leisure time physical activity and level of motor fitness). Three separate full models were created using stepwise straightforward regression with BMI, TST and %FAT as dependent variables. The highest autonomous influence on BMI and %FAT was ascribed to age and family obesity resemblance, whereas variation in TST was explained by level of motor fitness, age, city as a place of residence until the age of 14 and family obesity resemblance. Although the analysed variables explained only from 8% (BMI) to 13% (TST) of body fatness variation, indicating at the same time that most variations are explained by other variables, the impact of lifestyle family-shared factors on body fatness seems to be significant.