Red blood cell (RBC) aggregation was correctly reported in early 1768 as the increased deposition for blood to form spots of red in pathological conditions. However only recently have there been advances in technology to allow possible detection of RBC aggregation in vivo. Photoacoustic imaging (PA) shows a promising future in the detection of simultaneous in vivo RBC aggregation and oxygen saturation. This work presents, for the first time, the results of investigating aggregated RBCs using high frequency (> 20 MHz) photoacoustic (PA) imaging. Aggregation was induced by using 3% w/v concentration of 70 kDa Dextran and PA measurements were taken by a 25 MHz center frequency transducer. It was found that the spectral slope (SS) of the photoacoustic signals decreased by ~0.25 dB/MHz with RBC aggregation. The results are consistent with the findings of low frequency PA RBC aggregation study which also reported decrease in SS with increased aggregation.