SummaryMevalonic aciduria due to mevalonate kinase deficiency, an inherited defect of cholesterol biosynthesis, has presented with clinical variability in 10 patients from 7 families. We sought to define a genetic basis for this heterogeneity by determining mevalonate kinase activity in fibroblast heterokaryons obtained by polyethylene glycol fusion. To this end we developed a DEAE‐cellulose (Cl−) column chromatography procedure for assessing mevalonate kinase in cell extracts that would allow multiple rapid analyses. Fusion of control fibroblasts with those from affected patients from six families with mevalonate kinase deficiency yielded 37% of the mean control activity. None of the fusions between the six cell lines of patients resulted in measurable mevalonate kinase activity. Using the chromatographic procedure, we developed an optimized assay for mevalonate kinase in biopsied chorionic villi.Km values for chorionic villi were similar to those obtained in fibroblasts. Mevalonate kinase activity in biopsied chorionic villi showed a linear increase (0.75–4.3 nmol/min per mg protein) with gestational age from 7 to 14 weeks. Using the optimized assay in biopsied chorionic villi we performed a first‐trimester prenatal diagnosis in a pregnancy at risk for mevalonate kinase deficiency and correctly diagnosed an unaffected fetus. The availability of an optimized assay for mevalonate kinase in biopsied chorionic villi should allow reliable first‐trimester prenatal diagnosis for families at risk.