Streptococcus uberis and Staphylococcus aureus are important mastitis pathogens in dairy cows in the entire world. Recent publications showed that an extended therapy can be more efficient in combating such intramammary infections. The objective of this study was to evaluate the effect of an extended intramammary therapy to treat moderate and severe mastitis cases in lactating dairy cows under field conditions in northern Germany. From December 2005 to August 2007, a total of 157 clinical mastitis cases on 10 farms in northern Germany were enrolled in the study and randomly assigned (blocked by parity and body temperature) to one of two treatment groups (intrammammary lincomycin/neomycin 1.5-d (ALK) or 5-d (ALL)). Clinical, microbiological and cytomicrobiological cure rates were evaluated. Treatments were initiated before culture results. Cows were observed and evaluated on d 1 to 6,19 and 26. Six cases in 157 (3,8%) resulted in a therapy change in between 48 h after mastitis detection. Overall, treatments were not significantly different to controls regarding clinical cure rate. However, when the microbiological cure rate was evaluated, differences were observed. ALL appeared to be the most efficient treatment (54/55 [98%] P < 0.01, ALK 30/40 [75%]). This was due to the fact that ALL yielded increased cure rates towards mastitis caused by Streptococcus uberis which was the most frequent micro organism after coliform infections. We conclude that in farms with Streptococcus uteris mastitis, the 5-d extended lincomycin/neomycin treatment regimen was significantly more efficient in microbiological cure than the standard 1.5-d treatment.