We assessed the effectiveness of a 5-microm filter in reducing propofol-induced pain and determined whether any reduction is due to removal of contaminants or an alteration in flow characteristics. A total of 120 unpremedicated women (ASA 1-3, aged 18-70 yr) were randomly allocated to one of three equal-sized groups. In group A, propofol was drawn up and injected through an unfiltered plastic cannula. In group B, propofol was drawn up through a 5-microm filter needle and injected through an unfiltered plastic cannula. In group C, propofol was drawn up and injected through a 5-microm filter needle. Unmodified propofol from a 20-ml rubber topped vial at room temperature was used. A 22-g cannula was inserted into the largest visible vein on the dorsum of the non-dominant hand. Propofol was administered at 0.5 ml.s-1 and patients were asked about pain every 10 s until unresponsive, by a blinded observer. The pain score for the patient was the taken as the most severe pain documented. The frequency and severity of pain were similar among groups. We conclude that a 5-microm filter does not reduce pain associated with injection of propofol drawn from a vial with a rubber bung.