The optimum relative humidity for tomato pollination is generally thought to be 50-70%, but has not been well tested under controlled conditions or in combination with high temperatures. Starting two weeks before anthesis of the first flower, three tomato (Lycopersicon esculentum) cultivars, NC8288, FM-9 and FL7156, differing in heat tolerance were exposed to mild heat stress (31/25 vs. 28/22°C) at three relative humidity levels (30, 60 and 90%) in controlled environment chambers. These treatments represented vapour pressure deficits (VPD) ranging from 0.45 to 3.16 kPa during the day or 0.38 to 2.7 kPa averaged over a 24-h period. Pollen development in the anthers was followed cytologically, pollen release and germination were measured at anthesis, and seed production and fruit weight were measured in mature fruit. Fruit set, undeveloped flowers percentage, total fruit weight, and percentage pollen germination were most sensitive to high humidity (decreasing VPD) at high temperature, and were most sensitive to high temperature at high humidity. Over all, the optimum VPD was 2.25 kPa and plants grown at 28/22°C performed better than those grown at 31/25°C. Differences between cultivars in response to high humidity were less than those for temperature responses. Cytological examinations revealed pollen developmental anomalies in some, but not all, cultivars at 90 and 30% relative humidity.