Ketoconazole (KET) is active to control dandruff and seborrhoeic dermatitis. Objective assessments comparing the 1% and 2% shampoo formulations are scant. This open, randomized parallel-group trial was carried out to differentiate the effectiveness of KET 1% and 2% in severe dandruff and seborrhoeic dermatitis. A total of 66 patients with severe dandruff or seborrhoeic dermatitis were randomized to each of the two groups. A 2-week run-in phase was followed by a 4-week treatment phase, in turn followed by a 4-week follow-up. The efficacy of treatments was evaluated by combining squamometry X, Malassezia spp. counts and clinical assessments. After 2 and 4 weeks of treatment, KET 2% was significantly superior over KET 1% (p < 0.001) for decreasing both in flakiness and Malassezia density from baseline. The same trend was observed in the mean change from baseline in the overall dandruff severity score. Only 6 mild adverse events were reported. During follow-up KET 2% showed a trend to fewer relapses than KET 1%. KET 2% had superior efficacy compared to KET 1% in the treatment of severe dandruff and scalp seborrhoeic dermatitis. Biometrological evaluations were correlated with the clinical improvements and therefore useful to incorporate in future dandruff studies.