Linoleic acid is the main dietary essential fatty acid (EFA). To be fully utilized by the body, it must be metabolized to a range of other substances. The first step in this pathway is delta-6-desaturation to gamma-linolenic acid (GLA). This step is slow and rate-limiting, particularly in humans. If delta-6-desaturation is impaired for any reason, the supply of further metabolites may be inadequate for normal function. If the consumption of further metabolites is excessive, then a normal rate of delta-6-desaturation may be inadequate. In these circumstances the direct supply of GLA or further metabolites may be of value. This concept is illustrated by atopic eczema and diabetes, which may represent inherited and acquired examples of inadequate delta-6-desaturation.