Comparison of the patient records of the Finnish Red Cross National Cleft Center and the Central Register of Congenital Malformations showed a statistically significant yearly fluctuation as well as an increasing trend in the incidence of both cleft lip (and palate) and isolated cleft palate during the period 1948-1975. Almost significant difference was found in the monthly incidence of CL(P) with the peak in children born in April, while no such difference could be noticed in CP or both groups combined. The rapid increase and similarity of the incidence curves of both CL(P) and CP is suggestive of the action of simultaneous and similar exogenous agents in the pathogenesis of both types. In patients with CL(P) the etiological factor was considered to have been active 35 weeks and in CP 31 weeks before delivery. The monthly number of clefts was compared to the number of serologically verified infections (influenza A, respiratory syncytical, mumps, measles, rubeola, parainfluenza, adenoviruses as well as Mycoplasma pneumoniae) in the years 1971-75. No statistically significant correlation was found in this respect.