Three methods for the detection of Plasmodium falciparum infection in peripheral blood were compared during antimalarial treatment and follow-up in 32 Burundian patients: dipstick antigen capture assay, standard (TBF) and prolonged thick blood film examination (PTBF) (3 x 5 min and 3 x 20 min examination respectively). Parasitaemia was determined daily by comparison with total white blood cell counts (determined by Coulter counter) until no parasite was detected on 2 consecutive days by PTBF. Cumulatively, 231 observations were made with each assay: 64 were negative and 167 positive by PTBF (59 had parasite counts < or = 100/microL). Compared to PTBF, the sensitivities of TBF and the dipstick assay were 1.0 for parasite counts > 100/microL and 0.458 and 0.966 respectively for counts < or = 100/microL. Overall, the dipstick assay was significantly more sensitive (0.988 vs. 0.808; P < 0.001) but less specific (P = 0.013) than TBF. The dipstick assay is of potential use for monitoring response to drug treatment and for detecting low parasitaemias.