Both T helper IL-17-producing cells (Th17 cells) and regulatory T cells (Tregs) have been found to be increased in malignant pleural effusion (MPE). However, the possible imbalance between Th17 cells and Tregs, as well as the association of Th17/Treg and Th1/Th2 cells in MPE remains to be elucidated. The objective of the present study was to investigate the distribution of Th17 cells in relation to Tregs, as well as Th1/Th2 balance in MPE. The number of Th17, Tregs, Th1, and Th2 cells in MPE and peripheral blood was determined by using flow cytometry. The relationship among the number of Th17, Tregs, Th1, and Th2 cells was explored. It was found that the number of Th17, Tregs, Th1, and Th2 cells was all increased in MPE as compared with the corresponding peripheral blood. The number of Th17 cells was correlated negatively with Tregs in MPE, but not in blood. Th17 cells and Th17/Treg ratio were positively, and Tregs were negatively, correlated with Th1 cells, but not with either Th2 cells or Th1/Th2 ratio in MPE. This study supports earlier data that both Th17 cells and Treg are present at higher frequencies in MPE than in the autologous blood. For the first time, we show that Th17/Treg imbalance exists in MPE.