To evaluate severity of acute mucosal reactions (AMR) caused by conventional (CF) and accelerated fractionation (AF) regimens using a modified Dische system and to analyze differences in incidence and severity of AMR according to frequency and regularity of scoring. Sixty-six consecutive patients (33 CF, 33 AF) with head and neck cancer irradiated with 5 fractions in 5 days per week (CF) or with 7 fractions in 7 days (AF) to a total dose of 70 Gy. A modified Dische system was used for daily quantitation of AMR during radiotherapy until complete healing. Confluent mucositis (CM) was noted in 79% of patients in the CF group and 85% in the AF group. In 24% of the CF group and 18% of the AF group the CM presented a wave-like pattern. In 55% of CF and 67% of AF a classic triphasic pattern was noted. In 12 patients acute reactions did not transgress the level of spotted mucositis. The present study clearly shows that quantitation of the incidence and severity of acute mucosal effects strongly depends on frequent and regular scoring. A significant difference in the incidence of CM between the CF and AF groups was noted, mainly in weeks 4-6 of irradiation. When once-weekly irregular instead of daily scoring was evaluated, the incidence of CM was underestimated by approximately 20-36%. Acute mucosal reactions occur as a complex of morphologic and functional disorders with individual intensity, even among patients treated with the same fractionation regimen. In some cases they present a "wave-like" pattern during irradiation. Therefore, precise quantitation of acute effects requires regular and frequent scoring.