Effects of diseases on growth and yield of linseed (cv. Antares) were assessed by controlling diseases with fungicide treatments in field experiments at Rothamsted from 1988 to 1990. Fungicide seed treatments decreased the incidence of Alternaria lesions on cotyledons and stem bases of emerging seedlings in 1989 but decreased emergence in 1990. The incidence of leaf browning, associated with Alternaria spp. and Botrytis cinerea, was less in 1989 and 1990 than in 1988 when the period, in July, between flowering and harvest was wet, but the incidence of powdery mildew was greater in 1989 and 1990 than in 1988. Fungicide sprays decreased leaf browning in 1988 and powdery mildew in 1990. In 1988 there were more capsules per plant and the crop was taller than in either 1990 or 1989, when July and August were hot with periods of dry weather. The incidence of plants with dark brown stems, associated with Verticillium dahliae, was greater in 1989 and 1990 than in 1988, but was not greatly affected by fungicide treatments. The incidence of Alternaria linicola on sepals, capsule cases and seed from capsules sampled before harvest was decreased by fungicide treatments. Other fungi isolated from these tissues included A. alternata, A. infectoria, B. cinerea, Fusarium spp. and V. dahliae. Yields of grain and oil were increased by fungicide treatments in all three years, but the yield increases were greatest in 1988 when fungicide treatments also increased 1000-grain weight. The incidence of fungi on the harvested seed was not greatly affected by fungicide treatments.