An indwelling pleural catheter system comprises an indwelling catheter device (2) comprising a catheter tube (7) with a fenestrated distal end (7A) configured to reside in the pleural cavity of a subject and a connection hub (10) fluidically coupled to a proximal end (7B) of the catheter tube, a skin anchoring member (3) to anchor the connection hub (10) to the skin of the subject, and optionally a detachable ambulatory suction module (4) configured for detachable attachment to the connection hub (10). The suction module (4) comprises a fluid inlet (19) configured for fluidic coupling to the catheter tube (7) through the connection hub (10) and a fluid outlet (20) configured for detachable fluidic coupling to a pleural fluid drainage system (5) to drain pleural fluid through the detachable ambulatory suction module. The detachable ambulatory suction module is configured to exert a negative pressure in the catheter tube upon detachment of the pleural fluid drainage system from the suction module. Treatment of pleural effusion using the system of the invention is described. An indwelling catheter system for draining fluid from the peritoneal cavity, and methods of treating ascites, is also described.