Greenhouse growers and designers are repeatedly forced to choose between installations of natural ventilation (NV) or expensive fans with evaporative cooling (FV). The known advantages and disadvantages of each system do not presently allow a clear choice. To explore the effects on microclimate and crop, and to determine the limits of each system, experiments were done simultaneously in two adjacent compartments of similar area and orientation in which roses were grown; one naturally and the other mechanically ventilated with evaporative cooling. Three ventilation modes were studied in the naturally ventilated compartment: roof openings, side openings, and a combination of roof and side openings. Two fans capable of delivering a maximum ventilation rate of 0.06 m3 s-1 m-2 were used in the FV compartment. Consumption of electricity by the fans was monitored continuously as were micro-meteorological parameters in both compartments, e.g., wet and dry bulb air temperatures, leaf temperatures, radiation and soil heat flux. Yield (number of stems), stem length, bud diameter and length, leaf area index (LAI) and transpiration rate were measured to allow comparison between the treatments. In summer, the average air temperature, during the hottest hours of the day, between 10-14 h was higher in the NV house (31.8°C) than in the FV house (26.8°C), as expected. In addition, the humidity ratio, leaf temperature, transpiration rate and yield were higher in the NV house. Yield in the NV compartment was about 26-46% higher. However, the average stem length was larger in FV (43.4 cm) than in NV (38.7 cm). Relative humidity and LAI were lower in the NV than in the FV house. Small, however, significant differences between compartments were observed with regard to average bud diameter and length. During the hot days of the summer, plants in the FV house appeared to be more robust than in the NV house.