This study compared the efficacy of mechanical nasal lavages with pressurized seawater versus nasal irrigations with saline plus benzododecinium (antiseptic) plus oleosorbate (mucolytic). Twenty patients agreed to participate in a randomized, single-blind clinical trial. All patients underwent endoscopic endonasal ethmoidectomy for nasal polyps. The packing was removed after 48 h and patients were asked to start the same day nasal lavages three times a day. Clinical evaluations were performed: (1) by weighing residual nasal crusts and secretions after 21 +/- 2 days; and (2) by using visual analogue scales to daily record symptom scores. Data are presented as mean +/- SEM. T-test statistics for two independent groups were applied. The mean residual crust and secretion weights were 1,756 +/- 688 mg and 1,033 +/- 422 mg in the pressurized seawater group, 932 +/- 414 mg and 1,222 +/- 435 mg in the antiseptic-mucolytic saline group. No statistical differences were found. Sample size calculations showed that 100 subjects in each group would be necessary to confirm a 700-mg reduction in residual crusts in the antiseptic/mucolytic saline group (power = 0.80; two-sided type-I error = 0.05). Daily symptom score curves were similar in both groups and allowed us to give a description of post-operative complaints. The role of antiseptic, mucolytic and mechanical lavages in preventing post-ethmoidectomy crust formation is discussed.