A prospective study was carried out to assess the frequency of sudden infant death (SID) in the Brussels area. The study was conducted between January and December 1987 on infants presented to the health clinics of the Brussels "Office de la Naissance et de l'Enfance" (ONE). The infants entered the study if: (a) they were born between January 1 and December 31, 1987; (b) they attended an ONE consultation in the Brussels area between January 1987 and December 1988; (c) visits were repeated monthly during the first 12 months of life, or until death occurred; and (d) standard questionnaires were completed for each visit. For 4,064 infants these four criteria were fulfilled. During the study period 7 infants died of SID. Their deaths were unexpected and remained unexplained despite post-mortem investigations. For each SID case, 10 control infants were selected from the same health clinics. Controls matched the SID cases in the following respects: (a) dates of visits; (b) sex; (c) gestational age; (d) legal age; and (e) the profession of both parents. For most of the 65 items studied no significant difference was seen between the 7 SID infants and their 70 matched controls. Only two variables significantly differentiated the two groups. Repeated fatigue during feeding was seen in 4 of the 7 SID cases, but only in 10 of the 70 control infants (Fisher P = 0.017). Profuse sweating during sleep was reported in 2 of the 7 SID infants, and in none of the 70 controls (Fischer P = 0.007). These two infants' bedclothes were repeatedly wet with sweat.(ABSTRACT TRUNCATED AT 250 WORDS)