Modified Light's criteria are widely used to categorise pleural fluids as either exudates or transudates. These criteria include fluid/serum ratios and therefore require a blood sample. It is not clear whether analysis of pleural fluid alone would alter pleural fluid categorisation in routine clinical practice. In this study, pleural fluids were categorised using cut-off values for pleural fluid protein (=30 g/l) and lactate dehydrogenase (>0.45 of upper limit of the serum reference interval) recommended as providing optimal discrimination between transudates and exudates. The resultant categorisations were compared with those produced by application of modified Light's criteria. 77 pleural fluid specimens were included for analysis. Using modified Light's criteria, 32 fluids were categorised as transudates and 45 as exudates. Applying the pleural fluid cut-off values as outlined above, 26 fluids were categorised as transudates and 51 as exudates. Agreement was observed in 71/77 or 92.2% (95% confidence interval 86.2 to 98.2). It was concluded that in most cases, analysis of pleural fluid protein and lactate dehydrogenase alone produces the same categorisation as modified Light's criteria. Omission of a blood sample rarely affects the categorisation of pleural fluids in routine clinical practice.