Lysosomal acid lipase (LAL) is a hydrolase essential for the intracellular degradation of cholesteryl esters and triacylglycerols. This report describes a multi‐step procedure for the purification of LAL from human liver. After solubilization with non‐ionic detergent, acid hydrolase activity was purified 17000‐fold to apparent homogeneity by sequential chromatography on Concanavalin A Sepharose, carboxymethyl‐cellulose, phenyl Superose, Mono S cation exchange and Superose 12 gel‐filtration columns. This procedure yielded two silver‐staining protein bands of 56 kDa and 41 kDa on SDS/PAGE. Size‐exclusion chromatography of the 41‐kDa protein indicated that the enzyme was catalytically competent as a monomer of approximately 38 kDa. When assayed in the presence of cholesteryl oleate or trioleoylglycerol, purified acid lipase had V_max values of 4390 nmol fatty acid · min⁻¹· mg protein and 4756 nmol fatty acid · min⁻¹· mg protein⁻¹, and apparent K_m values of 0.142 mM and 0.138 mM, respectively. The purified enzyme was most active at low pH (4.5–5.0) and required non‐ionic detergent and ethylene glycol for optimal stability. Incubation of the 41‐kDa acid lipase with endoglucosaminidase H reduced the molecular mass by 4–6 kDa, demonstrating Asn‐linked glycosylation with high‐mannose oligosaccharides. Deglycosylation did not affect enzymic activity, indicating that carbohydrates are not required for LAL activity. Based on partial peptide sequence, an oligonucleotide was synthesized and utilized to isolate LAL cDNA clones from a human liver cDNA library. A full‐length LAL cDNA contained 2626 nucleotides and coded for a predicted protein of 372 amino acids, preceded by a 27 residue hydrophobic signal peptide. Hepatic LAL differed from fibroblast acid lipase at the N‐terminus and revealed extensive similarities with human gastric lipase and rat lingual lipase, confirming a gene family of acid lipases. Northern hybridization using the complete LAL cDNA as a radiolabeled probe indicated striking differences in mRNA expression among human tissues. LAL mRNA was most abundant in brain, lung, kidney and mammary gland. Placenta and HeLa cells expressed intermediate amounts of LAL mRNA, while RNA extracted from liver and heart showed low levels of expression.