diff --git a/fsl_glm.cc b/fsl_glm.cc
index eb1537d39c59b3334dd6f43f525583fa538ab296..0d78bab00a318cb12e82d176eeb3673b69bb1edb 100644
--- a/fsl_glm.cc
+++ b/fsl_glm.cc
@@ -57,6 +57,9 @@ using namespace std;
 	Option<bool> perfvn(string("--vn"),FALSE,
 		string("        perform MELODIC variance-normalisation on data"),
 		false, no_argument);
+	Option<bool> perf_demean(string("--demean"),FALSE,
+		string("        switch on de-meaning of design and data"),
+		false, no_argument);
 	Option<int> help(string("-h,--help"), 0,
 		string("display this help text"),
 		false,no_argument);
@@ -159,7 +162,8 @@ int setup(){
 		
 		data = tmpdata.matrix(mask);
 		voxels = data.Ncols();
-		data = remmean(data,1);
+		if(perf_demean.value())
+			data = remmean(data,1);
 		if(perfvn.value())
 			vnscales = Melodic::varnorm(data);
 	}
@@ -180,8 +184,10 @@ int setup(){
 	}
 
 	meanR=mean(data,1);
-	data = remmean(data,1);
-	design = remmean(design,1);
+	if(perf_demean.value()){
+		data = remmean(data,1);
+		design = remmean(design,1);
+	}
 	
 	if(normdes.value())
 		design =  SP(design,ones(design.Nrows(),1)*pow(stdev(design,1),-1));
@@ -252,6 +258,7 @@ int main(int argc,char *argv[]){
 			options.add(dofset);
 			options.add(normdes);
 			options.add(perfvn);
+			options.add(perf_demean);
 			options.add(help);
 			options.add(outcope);
 			options.add(outz);