| Aetiologies | Patients without pleural effusion# | Patients with UPE¶ | Patients with E/CPE+ |
| Gram-positive cocci | 1221 (32) | 235 (38) | 168 (64) |
| Streptococcus pneumoniae | 1178 (388/790) | 224 (75/149) | 124 (102/22) |
| Staphylococcus aureus | 20 (14/6) | 4 (3/1) | 6 (5/1) |
| Streptococcus viridans | 20 (20/0) | 6 (6/0) | 34 (34/0) |
| Other | 3 (2/1) | 1 (1/0) | 4 (4/0) |
| Anaerobes | 0 | 0 | 25 (10) |
| Gram-negative bacilli | 280 (7) | 47 (8) | 15 (6) |
| Haemophilus influenzae | 181 (17/164) | 25 (6/19) | 5 (3/2) |
| Moraxella catarrhalis | 23 (2/21) | 8 (0/8) | 0 |
| Pseudomonas aeruginosa | 38 (26/12) | 5 (4/1) | 2 (1/1) |
| Escherichia coli | 18 (13/5) | 3 (2/1) | 2 (2/0) |
| Klebsiella spp. | 5 (2/3) | 3 (2/1) | 3 (2/1) |
| Other | 15 (9/6) | 3 (2/1) | 3 (3/0) |
| Atypical agents | 517 (13) | 85 (14) | 4 (2) |
| Legionella pneumophila | 222 (39/183) | 35 (9/26) | 4 (2/2) |
| Mycoplasma pneumoniae | 51 (51/0) | 14 (14/0) | 0 |
| Chlamydophila pneumoniae | 119 (119/0) | 18 (18/0) | 0 |
| Chlamydophila psitacii | 19 (19/0) | 1 (1/0) | 0 |
| Coxiella burnetti | 56 (56/0) | 8 (8/0) | 0 |
| Virus | 50 (50/0) | 9 (9/0) | 0 |
| Other pathogens | 11 | 2 | 1 |
| Mixed infections | 101 (3) | 19 (3) | 23 (9) |
| Unknown aetiology | 1726 (45) | 249 (40) | 73 (28) |
Data for subgroups of pathogens are presented as n (%); data for microorganisms are presented as total (definitive diagnosis/probable diagnosis). UPE: uncomplicated pleural effusion; E/CPE: empyema/complicated parapneumonic effusion. #: n=3,833; ¶: n=621; +: n=261.