To determine the relation between adverse perinatal events and an unexplained mid-trimester elevation in maternal serum alpha-fetoprotein (MSAFP) in an unselected population, 16,093 women with a singleton pregnancy delivered between January 1985 and November 1991, at the Queen Mother's Hospital, Glasgow, Scotland, were studied. The elevated MSAFP group (n = 606) was associated with an increased risk of preterm delivery (odds ratios/95% confidence intervals) (3.7/2.6-5.2), small or gestational age (4.5/3.3-6.1), intrauterine death (3.9/1.7-9.4), and placental abruption (3.2/1.5-6.7). The risks increased with MSAP concentration. An unexplained mid-trimester elevation of MSAFP is associated with increased risk of adverse perinatal events, thus acting as a non-specific marker o a "high risk pregnancy". Alterations to current antenatal management in this group may improve prognosis.