The authors reviewed a series of 110 surgical specimens of primary non-small cell lung carcinomas from the Department of Pathology at the Hospital Clinic, University of Barcelona Medical School, between 1987 and 1997. The sample included 25 squamous cell carcinomas, 60 adenocarcinomas, 14 large cell carcinomas, and 11 neuroendocrine tumors. Electron microscopic subcellular characteristics of the lung cancer cells were studied to define the squamous, adenoid, or neuroendocrine differentiation in each tumor. An immunohistochemical study for Cyclin D1 was performed in 96 cases. In 71 cases (65%) the author found a single ultrastructural differentiation, and in 30 cases (27%) ultrastructural differentiation was double: 25 adenosquamous and 5 adeno-neuroendocrine. In 3 cases a triple adeno-squamous-neuroendocrine differentiation was found. There were no cases of squamous-neuroendocrine differentiation. In 6 cases no differentiation of any kind could be found. Cyclin D1 overexpression was found in 58% of all tumors. The positive expression rates in squamous cell carcinoma and adenocarcinoma were 72% and 62%, respectively. In purely adenoid-differentiated tumors there was a strong association between high Cyclin D1 overexpression and differentiation (p=.006). In bronchioloalveolar carcinoma the positivity rate was 70%; all were heavy expressers, compared with 25% of heavy expressers in adenocarcinomas as a whole (p<.005). In purely squamous tumors differentiated ultrastructurally no relationship was found between high Cyclin D1 expression and degree of differentiation (p=.08). Lung cancers are morphologically and molecularly heterogeneous, and certain molecular alterations are related to specific subcellular characteristics.