The occurrence of sulfonamide and macrolide antimicrobials, as well as trimethoprim, was investigated in conventional activated sludge treatment. Average daily loads in untreated wastewater correlated well with those estimated from annual consumption data and pharmacokinetic behavior. Considerable variations were found during a day, and seasonal differences seem to occur for the macrolides, probably caused by a higher consumption of these substances in winter. The most predominant macrolide and sulfonamide antimicrobials were clarithromycin and sulfamethoxazole, respectively. In the case of sulfamethoxazole, the main human metabolite, N4-acetylsulfamethoxazole, was included as an analyte, accounting for up to 86% of the total load in untreated wastewater. The results obtained illustrate the importance of considering retransformable substances, for example human metabolites, when investigating the behavior and fate of pharmaceuticals. Average concentrations of sulfapyridine, sulfamethoxazole, trimethoprim, azithromycin, and clarithromycin in activated sludge ranged between 28 and 68 microg/kg of dry weight. Overall the sorption to activated sludge was shown to be low for the investigated antimicrobials, with estimated sorption constants for activated sludge below 500 L/kg. Elimination in activated sludge treatment was found to be incomplete for all investigated compounds. In final effluents, the median concentrations for sulfamethoxazole and clarithromycin were 290 and 240 ng/L, respectively.