A patient with multiple myeloma presented with an accumulation of chylomicron-like particles. This rare finding resembled that of the type V hyperlipoproteinaemia phenotype. The lipid and lipoprotein concentration and composition were compared with values obtained from other patients with multiple myeloma, patients with the type V hyperlipoproteinaemia phenotype (accumulation of chylomicrons and very low density lipoproteins), and normal subjects. An immunoglobulin-lipid complex was demonstrated in our patient. This complex was found not to be associated with the chylomicrons and was detected only in the lipoprotein-deficient plasma. Lipid and lipoprotein concentration and composition differed from the other groups. Very low density lipoprotein concentration was reduced, and there was thus a marked difference from the type V phenotype. The chylomicrons derived from this patient were also richer in apolipoprotein C compared to chylomicrons derived from the patients with type V hypolipoproteinaemia. It appears that the abnormal composition of the triglyceride-rich lipoproteins observed in this patient renders her refractory to the normal pathways of metabolism.