Background: The objective of the study was to investigate the distribution of body mass index (BMI; kg/m2) among the working population in Flanders and the Brussels region of Belgium and the association of BMI with gender, age, occupation, smoking, blood pressure and, more specifically, with sick leave. Methods: Routine data on body height and weight were registered in 1994 in a cross-sectional way at periodical medical examination of employees in various industrial sectors. Results for 59,081 male and 47,414 female workers, aged 18–64 years, were registered on an optical form and stored into a database. Results: Descriptive statistics (mean ± SD) for BMI among males were 25.27 ± 3.92 (median = 24.86) and among females 24.03 ± 4.37 (median = 23.13). BMI was ≧25 in 48.5%, ≧30 in 11.1% and ≧40 in 0.3% of the males. Among females a BMI ≧25 was observed in 31.7%, ≧30 in 9.4% and ≧40 among 0.7%. Significant associations were found with age, occupation, smoking, hypertension and sick leave. In both sexes younger persons, clerical workers, smokers and persons with normal systolic or diastolic blood pressure had the lowest BMIs. More prominent in females, a trend of increasing year prevalence of sick leave with increasing BMI group was found. This association was not confounded by age, occupation or other variables in a logistic regression analysis. Conclusions: The high prevalence of a BMI ≧30, more pronounced in male than in female employees, needs further investigation. In addition, the cross-sectional association with sick leave needs confirmation in prospective studies.