To evaluate cost impact of prophylactic Neupogen (use in clinical settings, it is important to calculate appropriately current treatment cost incurred by severe neutropenia. OBJECTIVE: To collect resource utilisation and unit costs in Belgium for treatment of severe neutropenia of 5 chemotreated cancer types for which no reimbursement of prophylactic Neupogen(-use is given. These cancer types include multiple myeloma (MM), metastatic breast cancer, small cell lung cancer, non-small cell lung cancer and bladder cancer. To identify factors that may explain cost variation in treatment of severe neutropenia. METHODS: retrospective data collection of patient bills from 9 hospital centers spread all over Belgium over a period of one year (1/96-1/97) treated for severe neutropenia. Severe neutropenia was defined as neutropenic episode requiring hospitalisation. Only direct medical costs were considered from the perspective of reimbursement. Items collected included: hospital stay (duration and type); diagnostic procedures; drugs; transfusions; interventions; medical consults. RESULTS: 79 patient bills were collected. >75% of the registered neutropenic events occurred during first 3 ‘chemo’cycles. Total mean cost (35 Bfr = 1US$) was $4.918 (Med.: $4.529; 95% CI: $4.303-$5.533). DISCUSSION: Multiple regression analysis demonstrates that 2 factors (hospital duration and institution type) explain 85% of the cost variation. The institution factor may however mask a bias as all the hemato cancers (17%) with the more costly treatment procedures for neutropenia were treated in university centres. 85% of treatment cost is composed of only 3 factors: hospital stay (60%); drug regimen (15%) and lab tests performed (10%). CONCLUSION: treatment cost of neutropenia varies. Its spread is skewed to the right. It is appropriate to calculate geometric mean cost and to introduce specific cost estimates per institution type or per cancer type (hemato/non-hemato).