While there have been several thorough electron microscopic studies of the rat hepatomas, only a few human hepatomas have so far been studied by electron microscopy. 3Ve have been able to study the ultrastructure of the 18 cases of primary human hepatomas, conf- irmed light microscopica!ly at Yonsei University, College of Medicine, since Jan. 1973 to Sep. 1974. Even there was no specific or uniforrn ultrastructural abnormalities for the ma!ignant hepat- ocytes, several significant changes of the organelle from the normal hepatocytes ivere found. The size and shape of the nuclcus iias not uniform; the nucleus was large in 12 cases (I36.7 ;o,. And irregular in l.7 cases(94. 4' ." of the hepatornas. The inclusion body of the nucIeus was f'ound in 12 cases (6b. I%) The nuc!eus ivas well developed in all cases, and occasionally more than 2 nucleoli v;erc four.d in a nuc!eus. The significant abnormalities of .he cytoplasm in the malignant hepatocytes were endop!asmic I eticulum, and mitochondria; the rough endoplasmic reticulum was dilatated to form vacuoles, accompanied by detachment of the ribosome in ll cases (61. 1%) and the free ribosome was increased in 13 cases (i2. 2%!. The s!zocth endop!asmic reticulum was markedly proliferated in 11 cases C7i. 8oo'. The mitochondria shnv,ed tl.e mos irregular change: the number of the mitochondria was decreased in 8 cases (4. 4y,' j, increa, ed in 4 cases 22. 2: j and normal in 6 cases (33. 3% The loss of the cristae in the mitoch indria was noticed in 12 cases 66. 7Po j and dense particlcs in 11 cases (61. 1%). The glycogen was increased in 4 cases I 22. 2io) and the lipid in 11 cases (61. 1%! The lyso- some was well devetoped in ll cases and the filmentous myelin-!ike material vas found in 2 case.'.. On the cytoplasmic vall of the hepatomas, the desmosome was not found in 4 cases and the microvil!I v;-:; scatterred in G cases (3". 2.:o The 18 patients ivere divided into '2 groups by aipha-fetoprotein, positive or ncgative by double immunodiffusion techniqv.e and by Broader's pathological c!assification, class I and ]I and c'as g E; [( Th groups ivere compared each other v;ith elect-on r..icroscopic findings. The c!efinite cnnc!usions cou'.d not be obtained because of the uneven distributicn of the patier,ts hetween the groups, but there was a tendenc> that the inclusion body of the nucleus ivas more frcqucnt in AFP positive group and changes of the mitochondria, increment of free ribosnme and changes of cytoplamic meir.brane in AFP negative gr oup.