This study investigated environmental determinants of patterns and trends of occurrences of unstable malaria burdens in Kericho district of Kenya, a formerly malaria free highland district in early 1960s but classified by Kenya government in 2001 as malaria epidemic prone district where fatal malaria epidemics have reemerged seasonally each year since 1980s and represents a leading cause of morbidity and mortality in the district. To achieve this objective patterns and trends of malaria burdens; malaria hospital admissions, morbidity and mortality, climatic elements; and households' characteristics and district health delivery system were investigated. Secondary data on malaria burdens and climatic element were obtained from Kericho district health facilities and Kericho meteorological station respectively for the synoptic period 1988-2005. Primary households' characteristics were obtained using questionnaires from a randomly sampled household (N=301) apportioned to all four Agro-Ecological Zones (AEZs) study sites in seven administrative divisions based on 1999 national population census. Useful information on health efficiency delivery system in the district was obtained from key medical professionals respondents to questionnaires in Kericho district health facilities. The data were analysed displayed and interpreted using statistical methods; . tables, graphs, means, percentages, ranges, correlation, regression analysis and tests of significance (t-test) and Analysis of Variance (ANOVA). Tables and time series graphs, were used to study emerging patterns and trends in malaria burdens and climatic elements over time. Correlation and regression methods were used to determine the relationship between malaria burden and climatic and socio-economic data. The t-test was used to establish the significance of computed correlation coefficients between malaria morbidity and each of the household's variables. Archview Geographic Information System (GIS) software was used to create a malaria zonation map of the distribution of incidences of malaria morbidity in the district and SWOT analysis was used to analyse the effectiveness of malaria control, prevention and management algorithms in the district. Findings emerging from this study showed that outpatient malaria cases grew from 19643 cases in 1989 to 124408 cases in 1994 or a growth rate of 106.7% per annum and growth rate of 101.9% per annum in 1995-2002. Malaria hospital admissions grew from 10.27% per annum and a growth rate of 42.8% per annum in 1988-1994 and 1995-2002 respectively. Malaria deaths in Kericho district main hospital grew from 23 cases in 1988 to 34 in 1994, a growth rate of 6.9% per annum and in 1995-2002 malaria deaths grew from 84 to 160 cases or an annual increase of 18.1%