After detection of a few clinical cases of methaemoglobinaemia (methb) in our NICU, a prospective clinical study was undertaken to determine the extent of the problem and to identify the causes. Consequently, during the following 8 months all haemoglobin tests included simultaneous measurements of methb on an OSM 3 hemoximeter (Radiometer): 8% (n = 33) of 415 neonates were found to be methb positive (defined as > or = 6% methb). Mean methb was 19% (range 6.5-45.5%). Maximum methb concentrations were found on day 4-31 postpartum (mean 12 days) and the number of days with a positive methb sample ranged from 1 to 18 days (mean 6 days). About 40% of the neonates born at 25-30 weeks of gestation and 60% with a birth weight < 1000 g were methb positive. Also, there was a negative correlation between the size of the methb positive concentration and gestational age (r = -0.38, p = 0.02). Measurements of C-reactive protein and leucocytes, NADH reductase, pH, Cl, nitrate and nitrite were carried out in methb positive patients. The tests were repeated 1 week after cessation of methb. The only significant difference was an increase in NADH reductase at the second measurement. Likewise, a wide range of clinical parameters were registered and they occurred with a higher frequency among the methb positive patients when compared with a methb negative control group matched with regard to gestational age and the closest possible birth weight. The mean birth weight of methb positive patients was 1170 g and that of negative controls 1380 g (p < 0.006).(ABSTRACT TRUNCATED AT 250 WORDS)