The paper presents a comparison between CFD and experimental results with regard to flow patterns and ventilation rate of a naturally ventilated small greenhouse. The greenhouse was ventilated by side openings and roses were grown in it. Velocity vectors at several positions inside the greenhouse and near the openings were measured by a 3-D sonic anemometer. Ventilation rate was estimated from the decay rate of a tracer gas concentration and also was calculated from the product of the average air velocity near the openings by the area of the openings. To simulate the airflow within the greenhouse, three cases were investigated using CFD analysis. A very good agreement was obtained between the tracer gas decay method and the calculations using the 3-D anemometer data with regard to the ventilation rate. On the other hand, ventilation rates estimated by the CFD were usually noticeably higher. In some cases, the predicted detailed flow fields differed from the measured velocity vectors. Possible reasons for this disagreement are highlighted and discussed in the paper.