Partial deficiency in lipolysis usually results in only mild disturbances of lipid levels. However, when this is associated with impairment of the uptake of remnant particles and increased production of triglyceride-rich lipoproteins stimulated by environmental factors such as during normal pregnancy, chylomicronemia may ensue. We have previously reported a patient who had approximately 12% of normal LPL activity and developed severe chylomicronemia during pregnancy (Ma et al. 1993. J. Clin. Invest. 91: 1953-1958). Here we report four new patients with pregnancy-induced chylomicronemia. In the nonpregnant state, these patients had mild to modest elevation of triglyceride levels ranging from 80 to 623 mg/dl (0.9-7.0 mmol/l) but during the third trimester they became severely chylomicronemic with triglyceride levels ranging from 2314 to 14,596 mg/dl (26 to 164 mmol/l). Three of these four patients had partial lipoprotein lipase (LPL) deficiency. The molecular characterization of the LPL gene in these three patients with partial LPL deficiency revealed four novel unpublished mutations. Patient #1 is a compound heterozygote for Leu252Arg and Ala261Thr mutations which are associated with 25% of normal LPL activity. In addition, she has an apoE3/2 genotype. Patient #2 is a heterozygote for a Asn291Ser substitution with 69% of LPL activity and also has an apoE3/2 genotype, while patient #3 is a heterozygote for a Trp382Stop mutation with 54% of normal LPL activity and has an apoE4/2 genotype. The fourth patient (#4) with pregnancy-induced chylomicronemia does not have LPL deficiency and has an apoE3/3 genotype. The previously reported patient (#5) who had 12% of normal LPL activity due to homozygosity for a Ser172Cys mutation also has an E3/3 genotype. Our data suggest that mutations in the LPL gene that cause partial LPL deficiency might be a frequent factor in the pathogenesis of pregnancy-induced chylomicronemia.