In this double blind randomized placebo controlled clinical trial of filgrastim in chronic sinusitus, we analyzed costs of a 24-week interval in which filgrastim was administered. Since we hypothesized that the scheduled preventive visits within the trial might cause savings as compared to the regular situation in which these patients have a strong tendency to visit the outpatient clinic immediately in case of complications, direct medical costs within the trial were also compared to costs of regular treatment. The difference in costs between both trial groups was driven by the filgrastim costs (Euro 5108). If filgrastim costs were left out of consideration, no significant difference in direct medical costs remained between the filgrastim and placebo groups (Euro 2904 and Euro 2765, respectively). Indirect medical costs also showed no significant differences. Within a regular situation, costs of a 24-week interval were Euro 896. As filgrastim treatment had already been shown not to improve the quality of life, its cost-effectiveness in these patients can only be favourable in case of major clinical improvements. Furthermore, scheduled preventive visits in chronic sinusitis patients appear not to cause savings as compared to the situation in which patients are only seen in case of recurrences.