Poorly differentiated small cell carcinomas and malignant melanomas are among the rarest of primary colorectal malignancies. Although very aggressive and lethal, small cell carcinomas and the amelanotic variety of malignant melanomas have similar radiological features to adenocarcinomas. We present two different cases with these unusual tumours, with special emphasis on their dynamic contrast-enhanced magnetic resonance imaging features. The dynamic contrast-enhanced magnetic resonance imaging revealed that these tumours have a faster and stronger enhancement pattern than most adenocarcinomas. The technique is thus potentially useful for the preoperative diagnosis of unusual aggressive tumours and for guiding the choice of treatment.