Among lentil-growing regions, it is only in West Asia that both large-seeded, yellow cotyledon and small-seeded, red cotyledon lentil (Lens culinaris Medikus) are grown by farmers. This study examined the adaptation to temperature and rainfall of the two seed-size groups in West Asia; first in germplasm evaluated in Syria in the 1978/79 and 1979/80 seasons and assessed for cold susceptibility in Turkey in the 1979/80 season, and second in breeding lines over the decade 1984-93 at three sites in West Asia.Large-seeded material consistently had a longer reproductive growth period than the small-seeded group by 2.8 days, an extended period being required to fill its greater seed mass per pod. In the breeding material, the large-seeded group consistently produced taller plants and more straw than the smaller-seeded group. Germplasm with large seeds was less susceptible to winter cold than that with small seeds. Additionally, in the breeding material there was an advantage in seed yield of the large-seeded group over the small seeds in average temperatures <10 degrees C from January to April, with the converse true at higher temperatures. In the breeding material, the large-seeded group showed an advantage in seed yield over their small-seeded counterparts at the two wetter sites, whereas the small-seeded group was better adapted to dry environments. Based on this adaptation, the potential of some regions currently growing exclusively large-seeded lentil for producing the other seed type is emphasised.