Fibrates or PPAR alpha agonists, in particular fenofibrate, are known to increase homocysteine levels (Hcy). A 3 to 5 micromol/L increase in Hcy is commonly observed within the first few weeks of fenofibrate treatment; it then persists in plateau when treatment is continued and is reversible upon its cessation. Since its description in 1999, this pharmacological effect attracted a great deal of attention as epidemiological studies in most populations have shown that elevated Hcy levels i.e.Hcy> or =15 micromol/L are associated with an increased risk of cardiovascular events (CVD), venous thromboembolic events (VTE) and possibly cognitive disorders and bone fractures. Chronic kidney disease is also associated with elevated Hcy levels and since fenofibrate increases creatinine levels by about 10-12 micromol/L, a relationship between Hcy and creatinine was postulated. Animal studies have shown that the Hcy increase is PPARalpha dependent but to date animal or human studies have not provided a clear mechanism. In particular, fenofibrate treatment does not change vitamin B levels; however, vitamin B supplements reduce fenofibrate-induced Hcy elevation but not the concomitant cysteine elevation. Similarly, the increase in creatinine with fenofibrate only partially accounted for by a reduction in glomerular filtration rate (GFR) since creatinine production is also increased by 5-10%. In the FIELD study, a placebo-controlled study in 9795 patients with type 2 diabetes, fenofibrate over 5 years reduced non-fatal cardiovascular events and microvascular events such as albuminuria, the need for laser treatment for proliferative retinopathy or maculopathy and amputations but did not reduce fatal events. The increase in Hcy was indeed much larger that what would be explained by creatinine elevation and independent from baseline kidney function. Although baseline Hcy and creatinine levels were associated with subsequent risk of CVD, as suggested by epidemiology, their respective elevation was not. Of interest, after withdrawal of fenofibrate, a potential renoprotective effect was unmasked, as estimated GFR was 5 ml/min/1.73 m2 higher in previous fenofibrate-allocated patients than in previous placebo-allocated patients. There was no suggestion that Hcy elevation was associated with VTE (which were increased by an unknown mechanism) or bone disorders. In conclusion, the discrepancy between the role of baseline Hcy levels in epidemiology and the absence of effect when altering its levels by either decreasing them with vitamin B supplements or increasing them with fenofibrate, suggests that the risk factor(s) behind homocysteine should be found. Nevertheless, other studies are also needed to understand the mechanism and the implications of the moderate homocysteine and creatinine elevations with fenofibrate and other PPARalpha agonists.