A hypobetalipoproteinaemic kindred is described in which the proband manifested the clinical and biochemical features of the homozygous state. Unlike the apparent complete absence of apolipoprotein B in the plasma of the five cases of homozygous hypobetalipoproteinaemia reported so far, we were able to demonstrate minute quantities of this protein (approximately 0.025% of normal) in the plasma of the proband. This finding suggests that the disorder may not result from a structural gene defect but may rather reflect a failure of secretion.