Serum alkaline phosphatase (ALP) and its isoenzymes - liver (L-ALP), fast liver (F-ALP; also called high-molecular-weight ALP), bone (B-ALP) and intestinal (I-ALP) - were measured by agarose electrophoresis after pretreatment of sera with neuraminidase in 100 patients with Crohn's disease (CD) and 100 age-matched healthy controls. Total ALP was increased in 25 patients whereas the isoenzymes F-ALP, L-ALP, B-ALP, I-ALP were increased in 64, 84, 16 and 8%, respectively. But also in patients with CD and with normal ALP the isoenzymes were high in 39, 15, 1 and 1%, respectively. Total ALP, L-ALP and F-ALP correlated with gamma-glutamyltransferase (GGT) (tau = 0.22, 0.20 and 0.26, respectively, p < 0.05), but F-ALP also correlated with parameters of disease activity such as Crohn's disease activity index (CDAI), orosomucoid (OM) and erythrocyte sedimentation rate (tau = 0.28, 0.23 and 0.37, respectively, p < 0.001). In 38 patients, ALP and isoenzymes were controlled within 6 months. In these patients there were correlations between changes of total ALP and F-ALP with changes of inflammatory activity such as CDAI (tau = 0.31, p = 0.07 and tau = 0.30, p = 0.01) or OM (tau = 0.26 and 0.25, p < 0.05). Therefore, ALP seems to be influenced by inflammatory activity mainly via F-ALP which could be a marker of disease activity.