Twenty patients with Niemann-Pick disease (NPD) type B (4 to 42 yrs) from 18 different families who had each been diagnosed by the demonstration of reduced acid sphingomyelinase (ASM) activity in peripheral leukocytes were serially evaluated to: 1) characterize the clinical phenotype; 2) determine the plasma lipid levels and 3) determine the ASM genotype. Molecular analysis for the common NPD type A (R496L, L302P and fsP33O) and B (delR608) mutations was performed using genomic DNA from each patient. No patient had any of the type A mutations, whereas four patients were heterozygous for the common type B mutation, and three were delR608 homozygotes. Sequencing of unknown alleles identified five new mutations (P323A, C431F, Q292K, H575L, and H425R), two of which occurred in delR608 compound heterozygotes. Clinical evaluation demonstrated hepatosplenomegaly in all patients, with the exception of two adults who had undergone splenectomy and only had hepatomegaly. Fourteen patients were under the age of 18, and 12 had growth retardation (height and weight < 5th%). Liver function, assessed by SGPT and SGOT levels, was minimally impaired in several patients. Ophthalmologic examination revealed a macular halo or red spot in 10 patients, all of whom had mild neurologic findings (e.g. peripheral neuropathy, learning disabilities etc.). No ophthalmologic or neurologic features were present in any of the seven patients with one or more copies of delR608 suggesting a protective effect for this mutation. Determination of lipid levels revealed that all patients had abnormal profiles, regardless of genotype, which were characterized by total cholesterol and triglyceride levels above the 95th percentile for age, HDL fractions less than the 5th percentile, markedly decreased ApoA1 levels and increased ApoB levels. Echocardiography, stress tests and cardiac evaluation in five adult patients revealed mild mitral valve regurgitation in three, but no other abnormalites. Treatment of the hyperlipidemia in two adult patients by diet modification resulted in significant decreases in the lipid levels, but not into the normal range. In the pediatric patients, diet modification also reduced lipid levels, but had an adverse impact on growth. Efforts are now being directed to achieve diets that provide high caloric intake while restricting cholesterol to determine the efficacy of this approach. Thus, these studies provide the first genotype-phenotype correlations in NPD type B, particularly with regard to the ophthalmologic and neurologic features.