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Several studies investigated the effects of thoracocentesis on aspects of respiratory function without generally ensuring absence of coexistent lung pathology or homogeneity in initial size of the effusion.We studied 90 patients aged 61.6±15.9 years (mean±SD) separated into a group A with small-sized or medium-sized effusion (A=56 patients) and a group B with large and massive one (B=34 patients). There was no significant lung lesion or cardiovascular pathology. The basic spirometric parameters and maximal respiratory pressures were recorded on three instances: just before thoracocentesis (T1), 30 min after completion of the procedure (T2) and after 48 hours (T3).At T2 vs T1, groups A and B respectively presented significant change (mean±SD) (increase) in forced vital capacity (FVC) of 0.071±0.232 and 0.139±0.224 L, in forced expiratory volume in 1 s (FEV1) of 0.127±0.231 and 0.201±0.192 L, in FEV1/FVC of 2.8% and 4.9%, in peak expiratory flow rate (PEFR) of 0.342±0.482 and 0.383±0.425 L/s, in maximal expiratory pressure (MEP) of 0.049±0.037 and 0.049±0.039 kPa and in maximal inspiratory pressure (MIP) of 0.040±0.041 kPa only in group A while decrease in MIP with significant change of 0.055±0.051 kPa in group B. At T3 vs T2 in groups A and B, there was significant change (decrease) in FEV1/FVC of 2.7% and 4.6% as well as significant change (increase) in MIP of 0.036±0.046 and 0.115±0.060 and in MEP of 0.049±0.043 and 0.070±0.048 kPa.Thoracocentesis is associated with progressive-small relative to the volume of fluid removed-increases in lung volumes. In larger effusions at T2, a transient decrease in MIP is observed presumably due to temporary geometric distortion of the diaphragm immediately after fluid removal.