Twenty obligate carriers of infantile hypophosphatasia (HOPS), a severe autosomal recessive metabolic bone disorder, were studied and compared with 36 controls. Decreased serum alkaline phosphatase activity and increased urinary phosphoethanolamine excretion were confirmed in the HOPS carriers. Relative hyperphosphatemia was documented for the first time in the carriers. Logistic regression analysis was used to develop models for the diagnosis of and screening for HOPS carriers in the high-risk population of Manitoba Mennonites. Models based on serum alkaline phosphatase activity and on serum phosphate levels with or without urinary phosphoethanolamine excretion were used for diagnostic purposes. A model based on serum alkaline phosphatase activity and on the serum phosphate level was the most suitable for screening.