In patients with severe congenital neutropenia (SCN), the absolute neutrophil count (ANC) is raised during treatment with granulocyte colony-stimulating factor (G-CSF), resulting in a marked reduction of bacterial infection. Some patients, however, still have recurrent but less severe bacterial infections and severe periodontal infections. As it has been suggested that the biological activity of glycosylated recombinant human G-CSF (rHuG-CSF, i.e. lenograstim) is higher than the non-glycosylated form (i.e. filgrastim), we compared the two given in equimolar doses. Seven SCN patients participated in an open, randomized, double crossover study comprising 60 weeks, with four 12-week periods when the two drugs alternated after a 12-week run-in-period. The mean ANC values, sampled every second week, were 5·1 × 109/l during filgrastim treatment and 4·2 × 109/l during lenograstim treatment (P = 0·042). The ANC levels were also significantly higher during filgrastim treatment, when comparing each complementary pair of ANC measurements (P = 0·011) as well as the mean ANC values during each 12-week treatment period (P = 0·033). There were no differences regarding the frequency of infection, antibiotic treatment, gingival bleeding and the number of hospital admissions between the groups. We conclude that filgrastim and lenograstim displayed equal clinical efficacy, but that ANC levels were higher during filgrastim treatment, when administered in equimolar doses.