A survey to investigate the distribution and occurrence of cassava brown streak viruses, the causal agent for cassava brown streak disease (CBSD), was conducted in major cassava growing areas of Western Kenya in November 2011. The highest mean disease incidence was in Busia County (11.5%) while the lowest incidence was in Homa Bay County (1.5%). High mean severity score (3) was observed in farmers’ fields in Busia (Mungatsi, Matayos and Mundika divisions) but all the other areas showed no foliar symptoms. CBSD incidence correlated positively with disease severity on the leaves (r= 0.7, p< 0.05) and stems (r= 0.9, p<0.05). Polymerase chain reaction (PCR) detected single infections of CBSV and UCBSV in 64% and 3.4% of the positive reactions respectively. The study revealed a first record of Ugandan cassava brown streak virus (UCBSV) in Western Kenya. Mixed infections of CBSV and UCBSV were not found in any of the samples from the surveyed fields. The widespread occurrence of CBSV in Western Kenya has implications in the management of the disease in the region.