Objectives: Work in greenhouses is performed in warm microclimate during the most time of the year, involves usually moderately intense or heavy work. The working conditions in greenhouses might involve also indirect exposure to pesticides resulting from contact with pesticide-treated flowers and vegetables. The aim of the study was to investigate whether the work in greenhouse during pregnancy adversely influenced infant birth weight and, if so, which of the two main potential hazards typical for such environment (heavy physical work or exposure to pesticides) played the major role in this process. Methods: The list of 14 major greenhouses (each above 5 ha) growing vegetables (cucumbers and tomatoes) was obtained from the Polish Chamber of Horticulture. Between January 2001 and December 2003, 460 women at the age below 45 years, married or who lived with a partner and who had been working for a period of at least 2 years in greenhouses in Poland were asked to participate in the project. We classified pregnancies of women working in, and out of, greenhouses on the basis of energy expenditure during mother’s work into three groups: A (200–700 kcal/shift); B (701–1000 kcal/shift); and C (1001–1200 kcal/shift). Information about application of pesticides in 1997–2001 was received from persons responsible for chemical protection in each examined greenhouse. Trade names of pesticides, names and amounts of the active ingredients, type of cultivation and its area were abstracted from pesticide application registers run by each greenhouse operator. Pesticides were classified as reproductive and developmental (RD) toxins according to Pan American Pesticide Database classification. Results: The mean birth weight of infants whose mothers worked in greenhouse during pregnancy (work expenditure >1000 kcal/shift) was 177 g lower than that of those whose mothers worked out of greenhouse (light work <700 kcal/shift) (p = 0.05). Mothers who during work in greenhouse were potentially exposed to RD pesticides, delivered infants with birth weight lower by about 70 g. than infants’ mothers not working at places where pesticides RD were applied, but these findings were not statistically significant. Conclusion: Our results indicate that infants of mothers performing heavy work inside greenhouse during pregnancy had lower mean birth weight than infants of mothers working out of greenhouse. No similar effects of current exposure to pesticides was observed.